128 ADAPTATIONS. OECOLOGICAL CLASSIFICATION sect, in 



beneath the veins of leaves in land-plants, below or in the epidermis, and 

 is more extensive the drier the climate, or the more transpiration is favoured ; 

 by the environment. Parts of the fundamental tissue are developed as 

 mechanical tissue in stems of some plants, including Restiaceae.^ Stone- 

 cells and mechanical cells are differentiated in the chlorenchyma more or 

 less as idioblasts of various forms, which Vesque ^ distinguishes by such 

 terms as ' proteoid ', 'oleoid', and the like; they occur in leaves of 

 Proteaceae,^ Rhizophora,^ Restiaceae, Olea europaea (as long, sinous, 

 sclerenchyma-cells, which interweave and run both parallel and perpendi- 

 cular to the surface), Thea, and others. In some cases the utility of 

 these cells with thick, lignified walls is obvious ; it is to prevent the 

 shrivelling, collapse, or distortion of the vitally important chlorophyl- 

 containing tissue that would otherwise take place when the plant-member 

 withered. The strong construction of the epidermis in sclerophyllous 

 plants also performs the same service. 



In the production of thorns xerophytes show also their tendency 

 towards lignification. It has been recognized that plants living in 

 deserts and similar places are very thorny, and possess stiff, spiny or 

 prickly leaves, thorny stems, and the like. Such plants are characteristic 

 of the scrub in Australia, of stony steppes and high plateaux in Asia 

 (the ' Phrygana-vegetation ' of Theophrastus), the Kalahari, the deserts |" 

 of Egypt and North America, and others. Thorns vary widely in their ; 

 morphological nature, and may represent complete leaves or portions 

 of these, or emergences and prickles, or hgnified stems which are 

 vegetative axes or flower-stalks ; in accordance with these features various 

 growth-forms for example Grisebach's ' thorn-shrub ' and Reiter's j 

 ' thistle-form ' have been defined by different authors. 



Thorns, according to Lothelier's ^ researches, are evoked by dryness , 

 of atmosphere ; species such as Berberis and Crataegus, which are thorny ; 

 in dry air, become thornless in moist air. It has long been known that 

 spiny plants often lose their spines under cultivation on improved soil.*^ 



Nearly all those who have investigated the subject express the view 

 that, while thorns play no direct part in assimilation, they can hardly 

 be regarded as useless organs, since they presumably serve to protect the ; 

 plant against animals."^ Wallace ^ points out that thorny shrubs are I 

 especially abundant in those parts of Africa, Arabia, and Central Asia 

 where large herbivora abound. It seems to be beyond doubt that this 

 view is correct in certain cases, and that, for instance, the long spines of 

 Acacia horrida, A. giraffae, and other species in the dry tracts of South 

 Africa serve as defences against numerous wandering herds of ungulates ; 

 Marloth ^ calls attention to specialized adaptation exhibited by certain 

 species in that the longest and strongest spines occur on young individuals 

 or on root-shoots, which are most accessible to animals, while the branches 

 subsequently produced on tall trees are quite devoid of spines. A similar 

 phenomenon is witnessed in connexion with Ilex Aquifolium, the upper 

 leaves of which are usually not prickly when once the plant has grown , 



' Gilg, 1 89 1. ' Vesque, 1882. * Jonsson, 1880. | 



' Warming, 1883. ' Lothelier, 1890. , 



^ See Henslow, 1894, p. 223 ; Vesque et Viet, 1881, 



' Delbrouck, 1875 ; Marloth, 1887. 



Wallace, 1891. * Marloth, 1887. 



