What is milk?

A discussion of this question caught TPP's attention while driving across town the other day.  It basically involved someone representing the dairy industry, and people who produced/consumed "plant milks", soy milk, almond milk, etc. (it's quite a list).  Milk/milky is one of those not very precise words that some people are trying to narrowly define usually because of an economic interest in doing so.  Milk in the dairy sense is a secretion of mammary glands, and while primarily cow, goat, camel, and others also produce milk, as do the rest of mammals, and to some extent monotremes as well.  
These milks are emulsions, complex mixtures of organics suspended in water. And generally they are milky, a somewhat whitish, liquid.  The word even gets used in astronomy; galaxia refers to milk and our galaxy, also referred to as the milky way, is because the disk of distant stars across our sky looks "milky" to the naked eye.  As a side note, a small neighboring galaxy is called Snickers because next to the Milky Way it's just peanuts. 
Now the plant milks are basically an emulsion made by grinding up seeds and extracting the cell contents to make a milky emulsion.  Although the liquid endosperm of a coconut (big seed), the coconut milk, is a similar but self-made emulsion.  
This still leaves another category not mentioned or even recognized by the people in this program. Lots of plants probably 7-8% produce some form of latex, a milky liquid, that in some species contains elastic and non-elastic molecules that can make gums and rubbers.  The plants that secrete such latexes are sometimes so labeled, like milkweed.  This is what the botanist thought of when asked what milk is, something secreted by laticifers, specialized cells or ducts.  These are usually considered protective secretions to prevent or limit herbivory.  Raw opioids are obtained from a gummy latex secreted by injured poppy capsules.  
So sorry dairy people you do not get to co-opt the word milk and prevent everyone else from using it (their actual mission).  It's just too general a descriptive term.

Castilla rubber

One of the trees that is fairly frequent along water courses here on the warm, wet side (east of the mountains) of Costa Rica is the native rubber tree (Castilla elastica – Mulberry family).  It’s not really very impressive and students who just go ga-ga over some little lizard look upon trees dully and fail to understand the importance of these rubber trees in ecology and in history.  Castilla rubber was an important rainforest product for Costa Rica about a century ago and the source of all those rubber balls the Aztecs used for sport.  However para rubber (Hevea brasiliensis – Euphorb family) became the predominant rubber of commerce, and castilla rubber sort of faded into history.  So to give the troops something to remember, TPP tapped a tree by puncturing the outer bark (hard) into the softer inner bark that has lots of latex secreting cells.  The wound was small but the amount of latex that oozed out in just a few minutes was quite impressive, and when it coagulates, it is quite elastic.  On to chicle. 

Lettuce - crunch time

It's crunch time for leaf lettuces. If you planted lettuce early in the season, then by now it's bolting, and since it's laticifers are maturing, it gets disgustingly bitter. Laticifers are the cells or ducts that produce latex, milky sap; hey, the genus is named Lactuca after all. This happens because the shorter nights of June induce a change in growth, a shift from juvenile vegetative growth to a mature reproductive growth, and lettuce is only edible in the juvenile form. One way to push the season a bit, is to plant some lettuce later, and then grow the late lettuce as fast as possible because this seems to prolong the juvenile growth phase the most, although some varieties respond better than others. In particular the oak leafed varieties seem best at retaining the juvenile rosette growth form, but you must watch it closely, ready to harvest at the moment you detect any bolting. After this, you just have to wait until August and start planting all over again.