CHAP. XXXIII SOME STRUCTURAL CHARACTERS 129 



into a tall tree. It is evident that spiny plants by reason of their armed 



nature may defeat unarmed species and become more widely distributed ; 



but for all this we are not entitled to assume either that thorns are a 



direct adaptation to animals, or that they could arise by natural selection 



in a country rich in herbivorous animals. For example, against what 



animals did the Cactaceae and Agaves of Mexico and the West Indies 



require to defend themselves when they were evolved ? Would heredity 



have preserved these useless characters throughout the vast periods of 



time that may have elapsed since ungulates, which have recently been 



re-introduced, abounded in these lands ? (It is incontestible that spiny 



structures are now of use to Mexican succulent plants in protecting them 



from ungulates during the prolonged dry season). Kerner ^ assumes 



that the Mediterranean region is rich in thorny plants because animals 



also abound, and that on high mountains the absence of thorny vegetation 



is associated with the greater poverty of animal life. But in arctic 



countries there are many herbivorous animals, including large ones such 



as the reindeer and musk-ox, which roam about in great herds, yet no 



thorns occur, obviously because the conditions of humidity prevailing 



here and on high mountains do not conduce to the production of thorns.- 



In the north-temperate moist climate there occur many thorny 

 growths the significance of which is at present obscure. This is likewise 

 true of the strong spines of many palms, including Astrocaryum and 

 Bactris, growing in Amazonian forests. 



There are other thorns whose definite utility can be demonstrated, 

 and such is the case with those on stems of certain lianes. 



The physiological reason for the strong development of lignified 

 constituents is still somewhat obscure. But intense light and vigorous 

 transpiration seem to be the causes. Vesque and Viet,^ and subsequently 

 Kohl,"^ and Lothelier ^ experimentally proved that mechanical tissue 

 increases when transpiration is greater. Cockayne found that in the 

 rhamnaceous Discaria Toumatou thorns are not developed in moist air. 

 Stahl,' Dufour,^ and Lothelier^ found that mechanical tissue is more 

 strongly developed in light than in darkness : etiolated plants are very 

 weak-stemmed. On the other hand, experiment showed that with an 

 increased supply of water there was a diminished production of wood 

 in the oak and Robinia, and a reduction in the development of mechanical 

 constituents among monocotyledons.^" 



STUNTED GROWTH. SCRUB. CUSHION-PLANTS 



It has already been mentioned " that lack of water, and strong 

 transpiration, induce stunted growth. Wind, deficiency of water, and 

 other conditions unfavourable to growth, bring into existence elfin-wood 

 scrub, heath-scrub, ericaceous shrubs with bowed branches, and such 

 growths as that of malformed and stunted Pinus sylvestris as it occurs in 

 north-eastern Germany. Dry soil and strong transpiration impart to these 



' Kerner, 1869. * Sec Warming, 1892; Hcnslow, i8g4 ; Cockayne, 1905. 



' Vesque et Viet, 1881. * I^ohl. 1886. 



' Lothelier, 1890. ' Cockayne, 1905. ' Stahl. 1883. 



* Dufour, 1887. Lotheiier, loc. cit. " Grabncr, 1895. 



" See pp. 29, i7. 



WARMING K 



