344 CONDUCTING SYSTEM 



the pressure is relaxed, these latex-tubes undergo a contraction amount- 

 ing to as much as 4 or 5 per cent, of their diameter. The elastic 

 tension of the walls, of course, constitutes a reserve of potential energy 

 that will cause the latex to flow towards any part of the system in 

 which a diminution of pressure may happen to occur. 



The conclusion that latex represents, in the first instance, a 

 mixture of plastic materials, is not invalidated by the undoubted occur- 

 rence of useless by-products of metabolism among the contents of 

 laticiferous tubes. It is difficult to say whether the particles of resin 

 and caoutchouc, to which the milky appearance of typical latex is due, 

 should be reckoned as plastic substances or as excretory compounds. 

 The dilution of the latex in starved plants suggests that the particles in 

 question are partially absorbed under special conditions ; but, as 

 Schwendener has pointed out, the latex-tubes in such cases often 

 contain plug-like masses, enclosing large numbers of the solid particles, 

 which are evidently produced by partial coagulation of the latex. At 

 the same time, it is improbable that the dilution of the latex is entirely 

 due to this cause. On the other hand, it seems unlikely that the large 

 amounts of solid matter which are deposited in the latex-tubes of many 

 tropical trees in the form of resin and caoutchouc particles merely 

 represent a heterogeneous mass of useless (metabolic) by-products. 

 Schwendener thinks that the particles in question may possibly perform 

 the mechanical function of preventing the relatively light oil-drops from 

 rising, and the relatively heavy starch-grains from sinking, in the watery 

 matrix, so that a given distribution of the various plastic constituents 

 in the emulsion may remain undisturbed, except in so far as it may be 

 modified by streaming movements. 



In conclusion, mention must be made of two subsidiary ecologically 

 important functions that are frequently performed by latex. The first 

 of these depends upon the circumstance, that latex readily coagulates on 

 exposure to air, and is thus excellently fitted to serve as an occluding 

 material in cases of mechanical injury. The other ecological function of 

 latex consists in the protection which it affords against animal foes, 

 especially when it is as often happens endowed with poisonous 

 properties. In many cases this protective function assumes such 

 importance, that it even affects the location and disposition of the 

 ultimate branches of the laticiferous system. In the genus Euphorbia, 

 the latex-tubes that traverse the mesophyll frequently push their way 

 towards both leaf-surfaces as far as the inner side of the epidermis, 

 where they extend tangentially in every direction. If small portions 

 of the epidermis are removed from leaves of Euphorbia Lathyrh or 

 E. Myrsinitcs, the existence of this abundantly developed subepidermal 

 laticiferous system is revealed by the adhesion of large numbers of its 



