264 INFLUENCE OF CORRELATION AND EXTERNAL STIMULI 



Cacteae, many prickly or thorny plants, although there are some, such 

 as species of Nassovia. A dry climate with strong insolation has 

 always been considered favourable to the formation of thorns, but 

 this assuredly cannot be the only factor, for the long prickles which 

 appear upon the under surface of the floating leaves of Victoria regia, 

 and to a smaller degree in Euryale ferox, could not have arisen through 

 the influence of drought. What is really wanted here is experimental 

 evidence, of which we have none, except that of Lothelier. I have 

 never been able to prevent the formation of thorns in plants of Ulex 

 which I kept for ever so long under bell-jars in a saturated atmosphere ; 

 the shoots, developing into thorns, became in moist air longer, and 

 developed their leaves more, but there was no retarding of the forma- 

 tion of thorns in my experiment. Lothelier figures the apex of the 

 plant which produced leafy shoots instead of thorns, but if he had culti- 

 vated the plant longer ] the apparent leafy shoots might have become 

 thorns. Lothelier's statements give me the impression that he has 

 selected single cases specially favourable to his interpretation, without 

 having undertaken a careful testing of the whole of the experimental 

 cultivation. In other words, I do not think that up till now any more 

 has been proved than that in moist air the formation of prickles and 

 thorns is retarded, there is not proof that it can be suppressed. That 

 the formation of thorns and prickles is dependent on quite definite 

 factors is shown by the case of Ilex Aquifolium, where prickles appear 

 only on the leaves of young plants, not on the leaves of the older 

 ones. One might bring this into conformity with the fact that in 

 older plants, in consequence of the stronger development of the root- 

 system, the nutrition, especially the water-supply, is better, and this 

 would then be a case similar to that of many other plants, in which 

 it has been established that they lose their thorns in good soil. 



E. SUCCULENCE OF THE LEAF. 



Whilst it has not yet been established with certainty that other 

 relationships of configuration are directly dependent upon water-supply or 

 transpiration, it is possible that the succulence of the leaves of many plants 

 which grow in localities deficient in water may be considered as belonging 

 to this category. Such succulent leaves serving for water-storage diverge 

 usually in their form from ordinary leaves, and it is possible, though 



1 In plants of Crataegus pyracantha grown in moist air, Lothelier found the lowermost twigs only 

 ended in a thorn, 'all the others, having been subjected for a longer time to the influence of the 

 medium, were terminated by tufts of leaves at the period when the plant was cut.' I will only say two 

 things regarding this: (i) many lateral shoots of the higher order developed, as is shown in the 

 drawing, even in darkness into thorns; (2) the lateral shoots of the first order had not grown out 

 ' at the period when the plant was cut.' 



