CHAP, xxxin 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 1 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. 2 



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, 3 and subsequently 

 Kohl, 4 and Lothelier 5 experimentally proved that mechanical tissue 

 increases when transpiration is greater. Cockayne 6 found that in the 

 rhamnaceous Discaria Toumatou thorns are not developed in moist air. 

 Stahl, 7 Dufour, 8 and Lothelier 9 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. 10 



STUNTED GROWTH. SCRUB. CUSHION-PLANTS 



It has already been mentioned 11 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 



I Kerner, 1869. * See Warming, 1892; Henslow, 1894; Cockayne, 1905. 

 3 Vesque et Viet, 1881. * Kohl, 1886. 



5 Lothelier, 1890. Cockayne, 1905. 7 Stahl, 1883. 



* Dufour, 1887. Lothelier, loc. cit. 10 Grabner, 1895. 



II See pp. 29, 37. 



WARMING -K 



