342 CONDUCTING SYSTEM 



period, whereas non- nitrogenous plastic materials, such as oil and 

 starch, disappear altogether. 



The above results led Schullerus to conclude, that latex in the first 

 instance represents a " plastic sap," and that the latex-tubes serve for 

 its conduction. Faivre had previously arrived at similar conclusions 

 from his observations upon the composition of latex in plants grown 

 under various abnormal conditions. If, for example, seedlings of 

 Trar/opogon porrifolius are kept in the dark, the latex soon becomes 

 watery, owing to a diminution in the amount of solid constituents. The 

 same effect is seen in plants grown in the light but deprived of carbon 

 dioxide. In either case, if the plant is subsequently permitted to 

 resume its normal photosynthetic activity, the latex soon regains its 

 ordinary consistency. Faivre accordingly concludes that latex originates 

 in the photosynthetic organs, and is transported thence in order to be 

 utilised for nutritive purposes in other parts of the plant. 



The views of Schullerus and Faivre have not escaped criticism. 

 Kniep, in particular, has recently objected to the conclusions of these 

 investigators on various grounds. He insists more especially upon the 

 fact that increased " wateriness " of latex does not necessarily indicate 

 any decrease in the absolute amount of solid matter present. The 

 change of consistency may in such cases be due to mere dilution, that 

 is to say, to the distribution of the same quantity of solid material in 

 a larger bulk of liquid ; the continued growth of a latex-tube may be 

 accompanied by a corresponding increase of liquid contents, even when 

 photosynthesis is in abeyance [whereas no addition to the solid contents 

 is to be expected under these conditions]. Kniep also states, that when 

 Euphorbia seedlings are kept in the dark for lengthy periods, the starch- 

 grains in the latex-tubes do not disappear, or at most undergo partial 

 solution after a very long time. The persistence of these starch-grains 

 certainly does not support the contention that they represent a store of 

 reserve material. On the other hand, it must not be forgotten that the 

 presence of starch-grains in the latex is a peculiarity of the Euphor- 

 biaceae, so that the behaviour of these bodies need not affect our 

 judgment as to the nutritive value of latex in general ; the significance 

 of these remarkable starch-grains is, in fact, a matter for separate 

 enquiry. Kniep himself does not altogether deny the nutritive value 

 of latex. 



The preceding discussion sufficiently emphasises the need for 

 further experimental research upon this subject. It should not be 

 forgotten that latex may not have the same value in different families 

 of plants ; while it may be mainly nutritive in character in certain 

 Natural Orders, its ecologically important protective action which is 

 further explained below may come more into prominence in others. 



