Food - Plant material
As far as survival goes, food is way down the list of priorities. Other things are more likely to kill you before you starve. Let's start with plants. Many survival books have a range of examples of plant foods of what is edible, what is toxic - and so on. This information is very region-specific. I'll put it simply, plant food is evil!
So why are plants evil? Put yourself in a plants place. Here you are, collecting nutrients, trying to store up enough resources to make more little plants. Of course animals want to eat you - and you can't bloody get away. So what do you do? Well, make yourself as nasty as possible to eat. I told you they were evil.
Plants are basically in an arms race with herbivores. They make themselves as nasty as they can, and herbivores work out ways to get around it. Now humans have been cultivating plants since at least 10,000 years Before Present which, in combination with traditional trial-and-error, means we are quite good at eating what we've either selectively bred to be edible, or not died in the process of trying. Why is this important? Humans only use a handful of available plants as food. Also - and this is VERY important, we have been breeding the nasty stuff out of plants since neolithic times. Many of the common plant foods we see in the supermarket belong to very nasty groups of plants. A wild plant that looks like one we buy at the supermarket could very well kill you.
This of course is not totally hopeful for relying on salad as survival food but on the balance, I have to confess identifying, collecting, and eating bush food is one of my (many) outdoor hobbies. Suffice to say, I'll avoid specifics and just focus on general dos and don'ts. If you are keen on bush foods - get local guide books or download information, print it and stick it into your Vueetuee. You never know when you'll need it. As a warning, check out reviews of books before forking out money on them. Reputable publishers such as the Peterson one pictured are probably OK. Some books are too botanically minded, region-specific, or in the case of many European guides focus too much on urban weeds. It's worth doing some homework first.
Sorry to be a bit of a killjoy. I just have a big mistrust of plant material. This is not because I'm sort of carnivorous, beer-swilling, football watching redneck (I prefer ice hockey...) I personally love wild plant food, but in this case its up to you to go through your own learning curve and get informed as to what's locally edible. If you find some good local guides, let me know and I can place a link on the links page.
When considering how much time to invest in foraging for plants to eat, there are a few things to take into account. First, at each trophic level only 10% or so of the energy contained in the previous level is fixed in the organism. In short, meat contains more energy per gram. There is a reason that cows eat (and re-eat cud!) all day, while lions can afford to hang around and sunbathe. You need to eat a lot of plant material to get the same calorific content as meat. It has been argued that you can survive on plant material alone if you know what you are looking for. I have tried in a few different countries/ecosystems, and this is marginally so IF you just happen to be in a lush temperate or tropical area, and smack-dab in the middle of flowering and fruiting season.
Plant material should cooked or soaked before eating. Soaking in running water can leach out toxins, and heat denatures many toxins. However this is not guaranteed. Cooking also helps break down cell walls, especially starch storage cells, which makes the carbohydrate available for digestion.
If you really feel the need to get your greenery content, let's revisit some basic biology and botany.
Warning: There are many exceptions and additions to the points below. I'll say it again - get yourself informed about local plants before you need to use the information.
There are a few blanket warnings to get out of the way - derived from a range of sources.
Signs of plants to avoid
Prussic (hydrocyanic) acid. Avoid any plant with the smell of bitter almonds or peaches. This is an indicator of prussic acid ie. cyanide. Cherry laurel has this smell. Crush the leaves and remember the smell. Do not eat or even taste-test anything (see below) with this smell.
Oxalic acid. Oxalate salts are present in a wide range of plants such as wild rhubarb. They are reasonably easy to recognise by a sharp, dry, stinging sensation on the skin or tongue.
Bitter or soapy taste. These indicate alkaloids, which can be a bit corrosive and nasty.
Milky sap. As a general rule, avoid plants with a milky sap. Dandelions are a notable exception.
Red plants. Among other things, red is an indicator of oxalic acid.
Fruit divided into 5 segments. This is one of Lofty Wiseman's Don'ts. I have to admit I have no idea what the botanical basis of this is, but hey - better safe than sorry.
Grasses and plants with tiny barbs. This is kind of a no-brainer. While not necessarily indicating poison, it would be bloody irritating.
Grain heads with pink, purplish, or black spurs. This one is from US Army FM 21-76. As with the 5-segment rule, I don't know what the botanical basis is.
Beans, bulbs, or seeds inside pods. Much as they may resemble domestic pulses, many species also have noxious toxins particularly in the seed husk. They must ALWAYS be cooked. A handful of red kidney beans, or lima beans if eaten uncooked can make you incredibly sick.
Three-leaved growth pattern. Another US Army FM 21-76 gem. As above, I don't know what the botanical basis is. But when it comes to avoiding poisoning, a little blind faith can be exercised in these situations.
Opposite composite leaf pattern. I don't know how general this rule is, but the New Zealand Tutu has this arrangement, with a vein around the inside edge of the leaf. It may be a NZ bush myth (actually, I've seen copies of the news reports but I'm withholding judgement), but it is reputed to have killed a visiting circus elephant. In humans tutu poisoning has nearly killed unsuspecting hikers and only been reversed by industrial-strength doses of barbiturates. Additionally, Derris has this type of leaf structure, and its root is used as an ichthyocide (fish killer).
Old and withered leaves. I mention this below. One of the secondary functions of leaves are repositories of noxious metabolic byproducts. The younger the leaf shoot, the more palatable it is likely to be.
Umbelliferous plants. For those non-botanists out there,
flowering plant taxonomy has from the time of Linneaus been based on the structure of the flowering parts (basically, plant sex. The original terminology from Latin is kind of quaint and coy). Avoid plants with umbrella-shaped flower heads. An important (but not the only) reason for this rule is that the hemlocks are in the Umbelliferae, and are the execution method of choice for Greek philosophers such as Socrates. For those of you who are botanists, yes - I know. Carrots, dill, parsnip, or parsley are in the Umbelliferae too, so not all are poisonous. But there you go. Avoid anything with leaves and/or flowers that look like these.
Any thing with black berries. By berries, I mean single fruits, not composite fruits like boysenberries. The reason? The Solanums and allies have black berries. These include Belladonna and Deadly Nightshade. Again, to appease you tree-hugging botanists out there, yes - potatoes and tomatoes are also in the Solanaceae. If something is clearly a potato, the tuber should be fine IF cooked. But avoid the leaves, fruits, and flowers.
Testing new plants
If you really feel the need to blindly taste the local flora, there is a procedure you should follow. This comes from a range of sources including Wiseman and the US Army:
General condition. Put simply, if a plant looks manky, slimy, and worm-ridden it is best to leave it alone.
Smell. Crush the portion of the plant (leaves, roots) you are interested in. If it smells of bitter almonds, this indicates prussic acid. Avoid like the plague. If it basically smells yucky, this will be due to chemicals in the plant. Avoid it.
Skin. Crush and rub the plant part onto a sensitive part of the skin such as on the underside of your upper arm. Needless to say, don't rub them on your lips, eyes. Don't pick your nose, and be careful what you touch when going to the toilet! If it burns, stings, numbs - ditch it.
Lips, tongue, mouth. So far so good. In stages of about 15 minutes each, progressively: Place some on the lips; Place a small portion in the corner of the mouth; Place a small portion on the tip of the tongue; Place a small portion under the tongue; Chew a small portion but do NOT swallow. If there is ANY discomfort, burning, soreness, ditch it. Back to the drawing board.
Swallow. Swallow a small amount and wait 5 or so hours. Don't eat anything else during this time, so you make be sure what is causing what. If in doubt, get the old finger down the throat and get it back out. We already know it's not corrosive, so throwing it up is fine.
Eat. If you have survived this far without incredible stomach pains, nausea, diarrhea and generally anything that makes you feel like crap (no pun intended), then it can be considered safe for eating. I would still suggest taking it easy.
Leaves are used by plants for photosynthesis. While photosynthesis generates sugars, these are rapidly transported to the rest of the plant. Leaves contain little nutritional content. Leaves are also used in another way by the plant. Plants don't go wee-wee! Metabolic byproducts accumulate in leaves, which are then shed. So many plants use the leaves as repositories of nasty chemicals. These chemicals also serve to deter browsing herbivores. In short, there is very little nutrition in leaves, and they can contain nasty chemicals.
Notwithstanding the lack of nutrition in leaves, leaf shoots and young newly germinated plants are a slightly different matter. As they are the actively growing part of the plant, they contain sugars and, being young, have not had time to accumulate the nasty chemicals. Shoots are worth exploring as a food source. Leafy plants and fern shoots are generally nutritious, but use the testing procedure below first. Restrict yourself to the young shoots, and rub off any hairs or sharp bits. Of special note are the growing base of palm fronds. These are very rich in carbohydrate and generally unprotected by chemicals. Removing the head will kill the plant. This is necessary in a survival situation, but be judicious if you are just experimenting.
Seeds and nuts
Nuts should be treated with caution unless you really know what they are - back to the old local knowledge caveat. Do your homework before you need it or, as I do, make it into a hobby and something to actively do when hiking along. Among some relatively safe staples are pine nuts, and classical nuts you might find in the supermarket such as walnut, pecan, pistachio, hazels, and chestnuts. In northern hemisphere boreal forests, beech nuts are a useful source of protein. If you just happen to be in tropical Australia, macadamia nuts are good too. Also, in many parts of Europe and northern America you may come across almonds (Prunus), but beware of these as they can also contain high levels of prussic acid. Remember the almond smell warning above? Acorns are notable in northern America and Europe. The nuts are edible, but only if you shell and soak them, changing the water frequently for around 3-4 days. After that time you can roast them. I get a bit paranoid and boil them then roast them. Squirrels must have stomachs of steel! This is all great if you are in northern America or mid-boreal Europe. Seasonality and geographical availability may mean that, like fruits, nuts are not going to be a reliable food source across all places and times.
Seeds should generally be avoided. Yes, humans pretty much survive on cultivated grasses, but we've been working on breeding good varieties of the good species since neolithic times. Even then we get sometimes get it wrong - the great Ergot outbreak springs to mind. If you want to gather grass seeds such as wild millets, you have to be prepared to gather a lot to make it worth you while. Seeds should be soaked, pounded - always apply the taste test before consumption. If there is any black discoloration on the seed - stay well away. This may indicate mould infection such as ergot which, apart from causing some pretty nasty hallucination, killed a gazillion people during the middle ages.
Coconuts however are great. The milk is nutritious, the flesh (which develops as the nut gets older) is good. However... in excess you can get coconut belly - abdominal discomfort accompanied by long visits to the toilet. The newly germinated nuts are very good too, with a spongy nutritious flesh.
I can't really provide any general advice about fruits that holds across different floras. My primary advice is to consult local guides for the areas you are likely to be in. A bit of botanical thinking should tell you why this is so. Fruits supply nutrients to germinating seeds, so it's in the plants interest that you not be able to eat them - for example the Solanums. However, some plants rely on frugivores (fruit eaters) to eat the fruits and disperse seeds in droppings. Herein lies the quandary - are they using fruits for culinary good or evil? In tropical regions fruits are less likely to kill you, and they are not too different from what you should be familiar with in the supermarket.
Roots and tubers
These contain energy storage materials ie. carbohydrates (sugars, starches) for the plant. Roots are a good food source to look for .General rule: All roots and tubers must be cooked. Some roots may contain toxins, but these are usually destroyed by cooking. A notable species to be careful of is Cassava. While it is a staple in parts of Africa, the tuber is toxic if eaten raw. It MUST be cooked. Unlike leaves, plants don't invest much energy in putting toxins into roots. They can also contain woody material (parenchyma), but this can be softened by cooking.
There are some poisonous roots out there - but there are no hard and fast rules to work out which ones they are. Avoid the roots of vines. The derris vine is found in southern America and Australia and is used as a pesticide - derris dust (rotenone). It's a metabolic inhibitor and will kill slugs/snails, it tends to hammer birds and mammals, and is widely used as a fish-killer by museum collectors. Needless to say, not fit for human consumption. It is best to do a bit of homework on the local flora, photocopy some pages from the books, and stick it into your VueeTuee.
To all you biologists out there - don't make me slap you! Yes, I know fungi aren't plants. But they kind of look like them. Regarding fungi as food - don't even bother. Wiseman is fond of them, but he's a fungus geek and runs courses on their identification. If you have local guides you might be able, with a lot of foraging, to get something that might not kill you. But you really have to know what you are doing - field mushrooms are in the same family as fly agarics, for example. Avoid old-wives-tale's about colour when cut, gills, and so on. The nutritional content of fungi is negligible, and the risk of severe poisoning or death is very high. Hey, I like mushroom foraging as much as anyone else. But I still wouldn't trust my identifications in a novel place or without a microscope.
If you are fortunate enough to be stranded on the seashore, many algae are edible. Notably are the commonly named sea lettuce (Ulva spp.) and lavers (Porphyria spp.). These are leafy species - green and reddish respectively - which are crisp, crunchy and kind of tasty. Green algae in general are edible. They have few noxious chemicals. As the algae gets browner and redder, they get more unpalatable due to tannins and polyphenolics. They won't kill you, but they taste kind of crappy. Very few fishes eat brown and red algae - there is a reason. If you are harvesting algae however, make sure they are taken from flowing waters, not high tide pools that get inundated only once a month.
I strongly recommend getting informed if you want to embark on eating wild plants. I do - it's one of my many outdoor hobbies. Unfortunately - botany being what it is- the guides are necessarily local in extent. Here are a selection of ones worth checking out.