2io INFLUENCE OF CORRELATION AND EXTERNAL STIMULI 



of food-material than when a fixed quantity of food-material is distributed 

 over a number of leaves. It is in consequence of this that leaves upon 

 stool-shoots are much larger than normal, and that organs which are 

 abortive on the ordinary leaves often appear upon them. Thus on stool- 

 shoots of Robinia Pseudacacia the stipelles develop into leaflets l , and 

 in Sambucus nigra the stipules which are commonly much reduced may 

 become leaf-like. 



The behaviour of the stipules of many plants is most instructive. 

 Their size depends upon the influence of the leaf to which they belong. 

 If the leaf-primordia are removed above the stipules at the earliest possible 

 moment a remarkable increase in size of the stipules takes place as the 

 following figures will show. Of two plants grown together in a pot from 

 equally heavy seeds, one had the leaves left untouched, in the other they 

 were removed at the earliest possible moment ; the surface measurements 

 of the stipules were as follows : 



Plant with leaves. Plant "without leaves. 



1. stipule 141 D mm, 239 Dmm, 



2. 172 5 6 * 

 3* * 5 



Occasionally we find in nature misformed plants in which the leaves 

 are completely aborted whilst the stipules are greatly enlarged, and the 

 latter condition is a consequence of the former. We are able readily to 

 recognize this in Lathyrus Aphaca whose leaves are transformed into 

 tendrils, whilst their usual function is performed by the uncommonly enlarged 

 stipules (see Fig. no). It is not however possible to prove such a corre- 

 lation in all plants provided with stipules, and we may perhaps account 

 for this by the fact that the stipules lose their capacity for growth at an 

 earlier period than can be reached by experimental interference. We cannot, 

 for example, promote an enlargement of the stipules in Phaseolus multiflorus 

 and other plants by the experimental method just referred to. 



I have already said that relationships of correlation have probably to 

 do with the unequal formation of the leaflets of compound leaves 2 . This 

 dependence certainly exists in the case of the cotyledons of Streptocarpus. 

 In this plant the two cotyledons attain very unequal dimensions, the one 

 remains small, the other becomes a large foliage-leaf lying upon the ground ; 

 if the large cotyledon be removed at an early period, or if its growth be 

 restricted by enclosing it in plaster of Paris, the other cotyledon develops 

 to take its place 3 . 



1 See Part II of this book. 2 See p. 127. 



3 F. Hering, Uber Wachstnmskorrelationen infolge mechanischer Hemmung des Wachsens, in 

 Pringsh. Jahrb. xxix. p. 142. 



