256 PHYSIOLOGICAL DEVELOPMENT. 



attempting to seek in these lower types for any more specific 

 interpretation of it, let us pass to the higher types. The 

 argument -will be amply enforced by the evidence obtained 

 from them. We \vill look first at the conditions which they 

 have to fulfil ; and then at the way in which the functions 

 and structures adapting them to these conditions arise. 



278. A terrestrial plant that grows vertically needs no 

 marked modification of its internal tissues, so long as the height 

 it reaches is very small. As we before saw, the spiral or 

 cylindrical rolling up of a simple cellular frond, or the more 

 bulky growth of a simple cellular axis, may give the requisite 

 strength ; and the requisite circulation may be carried on 

 tli rough the unchanged cellular tissue. But in proportion 

 us the height to be attained and the mass to be supported 

 increase, the supporting part must acquire greater bulk or 

 greater density, or both ; and some modification that shall 

 facilitate the transfer of nutritive liquids must take place. 

 Hence, in the inner tissues of plants we may expect to find 

 that structural changes answering to these requirements 

 become marked, as the growth of the aerial part becomes 

 great. Facts correspond with these expectations. 



Among the humbler Acrogens, which creep over, or raise 

 themselves but little above, the surfaces they nourish upon, 

 there is scarcely any internal differentiation : the vascular 

 and woody structures, if not in all cases absolutely un- 

 represented, are rarely and very feebly indicated. But 

 among the higher Acrogens the Ferns and Lycopodiums 

 which raise their fronds to considerable heights, there are 

 vascular bundles and hard tissues like wood; and by the 

 Tree-Ferns massive axes are developed. That the relation 

 which thus shows itself among Cryptogams is habitual among 

 Phaenogams, scarcely needs saying. 



Phacnogams, however, are not universally thus charac- 

 terized iu a decided way. Besides the comparative want oi 

 woody substance in flowering plants of humble growth, and 



