722 ECOLOGY 



sembling rods, dumb-bells, etc. As a rule latex is white, as suggested 

 by the common name of milky juice, but in the poppy family it may be 

 yellow, orange, or red. Latex elements are living, having thin plas- 

 matic layers along the walls, which commonly are thin and readily per- 

 meable to water and solutes, though in Euphorbia the walls are thicker 

 and pitted. 4 



In many plants (as in the common milkweed, Asclepias syriaca) latex flows copi- 

 ously from a wounded surface, indicating its great abundance and also the high 

 pressure at which it exists in the tubes; it soon coagulates upon exposure to air. 

 Latex is of great commercial value, as it is the chief source of rubber; the leading 

 rubber-producing plants are tropical trees of the nettle, dogbane, and spurge fami- 

 lies. Opium is a mixture of alkaloids from the latex of the poppy. 



The role of latex. The presence in latex of leucoplasts, proteino- 

 plasts, and elaioplasts (together with the starch, protein, and oil which 

 they form), as well as of proteolytic ferments, has led to the theory that 

 latex tubes are food reservoirs; this theory finds further support in 

 the fact that in rapidly growing young plants of Euphorbia Lathyris 

 the latex is very milky and is rich in starch, fats, and albuminous sub- 

 stances, while in old plants the latex is translucent and watery and is 

 poor in these substances; furthermore, in winter, albumin is scarce in 

 the stem latex and abundant in the root latex. Again, plants grown in 

 the dark or in air that is deprived of carbon dioxid have weak watery 

 latex without starch. However, the presence of such waste products as 

 caoutchouc, gums, resins, waxes, tannins, and alkaloids has led equally 

 to the theory that latex tubes are excretory reservoirs. The presence 

 of starch does not necessarily favor the food reservoir theory, since there 

 is little evidence that the starch is ever used; in starved plants, for 

 example, it suffers no appreciable decrease. There is some evidence 

 that the fats and proteins of latex are utilized as food. Very probably 

 latex tubes are general catch-alls, containing both surplus foods and 

 waste products; the latter generally are greater in amount, and it is 

 likely that the latex tubes are of significance chiefly as excretory reser- 

 voirs. 



Related to the food reservoir theory is the hypothesis that latex tubes 

 represent a conductive system, a hypothesis favored supposedly by the 

 continuity of the tubes and by the paucity of cross walls, as well as 

 by the intimate connection sometimes existing between the ends of the 

 tubes and the palisade cells. The adherents of the conduction theory 

 regard the latex tubes as paths of movement of carbohydrates and pro- 



