2 58 COOK 



are frequently apparent in the literature of the subject. Thus 

 it has been inferred from experiments on a spiny New Zealand 

 plant that the spines, instead of being a means of protection 

 against grazing animals, of which there were none in New 

 Zealand, are in reality an adaptation against transpiration, 

 because they do not appear when the plants are cultivated in a 

 humid atmosphere. 



"After being placed in the moist chamber, the plants devel- 

 oped no more spines and are now seedling plants in all respects 

 except for the few spines, which were developed prior to the 

 culture in moist air. Moreover, it seems evident that such 

 plants would remain in the seedling form so long as they were 

 kept in an atmosphere constantly moist and exposed to a feeble 

 light. 



" Even an adult shoot on a full grown plant in the open and 

 freely producing spines, may have any further production of 

 such suppressed at once, if the shoot should continue its growth 

 under slightly more hygrophytic conditions. Thus quite recently, 

 I observed on the clay hills near Wellington, a shoot creeping 

 near the ground whose apical portion was covered by grass. 

 This shoot where fully exposed to the light was spinous as usual, 

 but where shaded and in a slightly moister atmosphere was quite 

 without spines. 



" From the above it follows that the production of spines in 

 Discaria Toumatou can be controlled at will by specifically 

 changing its environment — a plant exposed to a dry atmosphere 

 and normal light producing spines, whilst one exposed to a moist 

 atmosphere and a feeble light produces no spines, but in their 

 place leafy shoots of unlimited growth. 



" That spines on xerophytic plants are an adaptation against 

 the attacks of grazing animals is a matter of such general belief 

 as to be admitted into certain botanical text-books as a proved 

 fact. 



" It seems, however, to me that my experiment, detailed above, 

 is a fairly crucial case, and that in Discaria Toumatou, at any 

 rate, the spines are a direct response to conditions of dryness, 

 and function as a special contrivance for checking transpiration. 

 If so, then they have nothing to do primarily with attacks of 



