282 SCIENCE PROGRESS 



between the amounts of laticiferous tissue and " conducting 

 parenchyma" of the leaf veins, such that a large development of 

 laticiferous tissue was accompanied by a comparatively small 

 amount of parenchyma. Examples given are Euphorbia myr- 

 sinitcs and E. glandulosa. The conduction performed by these 

 two tissues was accordingly held to be complementary in amount. 

 But it is scarcely safe to conclude from the complementary 

 development of these tissues that the function of conducting 

 assimilates is shared by them. The relatively smaller develop- 

 ment of parenchyma in the veins might arise equally well from 

 a smaller need for the mechanical support which it provides, for 

 such support may also be given by fibres, epidermis, shape of 

 leaf, etc. 



The Latex. — It is remarkable how many substances enter into 

 the composition of the latex. An analysis of that of Euphorbia 

 cyparissus, given by Wiesner and quoted by Kniep, showed it to 

 contain 72 per cent, of water. Of the dry material the com- 

 position per cent, was resin 57*5, gum 13, caoutchouc 10, 

 sugars and bodies soluble in ether 15, albumen '5 and ash 

 3'6. Starch, oils, acids, colouring matters, mineral salts and 

 enzymes are among other constituents of latex, and so it is seen 

 that both substances of use in metabolism and others often 

 regarded as waste products are to be found in the laticiferous 

 tubes. 



The presence of such materials as sugars and starch led 

 some observers to conclude that these are probably conducted 

 in the tubes, which appeared to be, in addition, receptacles for 

 waste products. For instance, Haberlandt gave it as his opinion 

 that "... the latex is primarily a formative sap " and that 

 "... the tubes function as conducting organs for it." 



Kniep, however, has pointed out that the plastic materials are 

 present in much smaller amount than are such substances as 

 resins, caoutchouc, etc. He considers that as these latter com- 

 pounds are formed with great expenditure of energy on the 

 part of the plant, it is only natural to suppose that their produc- 

 tion continues because it is of some definite biological value to 

 the plant. This value lies in the power of the latex to coagulate 

 rapidly on exposure to air, owing partly to the presence of 

 oxydases. In this way Kniep thinks that the chief function of 

 the latex is an " ecological " one, since it helps to close wounds, 

 a function thought by Haberlandt to be quite subsidiary to that 



