*.] XEROPHYTIC DEGENERATIONS 85 



diminished in size or are wanting, but the 

 tissues undergo corresponding degradations. It 

 would carry me too far to give many illustra- 

 tions of each sub-flora, but I must be content 

 with a comparatively few examples from which 

 the reader may safely generalise. 



In tropical dry " thorn forests " and open 

 deserts, wherever plants can grow at all, such 

 as the dry water-courses or Wadys of deserts, 

 water remaining at more or less great depths, 

 spinescence is of the commonest occurrence. A 

 true "spine" is an arrested branch terminating 

 in a hard and sharp point. I have already 

 alluded to it in the rest-harrow and Zilla, etc. ; 

 but instead of having its internal structure 

 adapted to carry water to the leaves and to 

 enable the shoot to elongate, as it ought, the 

 " mechanical " tissues, as the wood, solidify 

 and become intensely hard, the actual tip of 

 the spine which represents the soft, succulent 

 "growing point" of the shoot being as hard 

 as the wood fibres below. It is analogous to 

 the wood of the Kauri pine, described by Mr 

 Reid. This prevalence of spiny processes is 

 not confined to arrested branches, but occurs 

 in leaves (furze, cactuses) and stipules as well 

 as in acacias, etc. 



This universal presence of spiny degradations 

 is obviously i.e. t by inductive reasoning the 

 actual result of what Darwin called the " definite 



