THE INNER TISSUES OF PLANTS. 273 



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 will look first at the conditions which they 

 have to fulfil; and then at the ways 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 car 

 ried on through the unchanged cellular tissue. But in pro 

 portion as 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 require 

 ments become marked, as the growth of the aerial part 

 becomes great. Facts correspond with these expectations. 



Among the humbler Cormophytes, which creep over or 

 raise themselves but little above, the surfaces they flourish 

 upon, there is scarcely any internal differentiation: the 

 vascular and woody structures, if not in all cases absolutely 

 unrepresented, are rarely and very feebly indicated. But 

 among the higher types 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 

 Pha^n ogams, scarcely needs saying. 



Phsenogams, however, are not universally thus charac 

 terized in a decided way. Besides the comparative want of 

 woody tissue in flowering plants of humble growth,, and 

 64 



