CHAP, ii The Differentiation of the Tissues 219 



landt's Physiologische Pftanzenanatomie, which, written on 

 similar lines, went much further, and attempted a classifica- 

 tion of all the tissues of plants on a physiological basis. 

 Its aim was to show that the whole anatomical structure 

 and mode of arrangement of tissues, comprising not only 

 the entire plant, but the most minute subdivisions of it, 

 are illustrations of adaptation to physiological needs. 



He discussed first cells and their modifications and then 

 their combinations into tissues, of which he distinguished 

 the tegumentary, the mechanical, the assimilative, and the 

 conducting systems. In discussing the mechanical arrange- 

 ments he followed the lines of Schwendener, but carried 

 them further through the vegetable kingdom, showing that 

 such arrangements can be distinguished even in the larger 

 Fungi. 



His treatment of the conducting tissues differed greatly 

 from that of the morphologists. He grouped together 

 under the one name the parenchyma of the cortex, pith, 

 and medullary rays, the vascular tissue, and the laticiferous 

 cells and vessels. Dealing in more detail with the vascular 

 bundles, to which he gave the name Mestom, he renamed 

 the xylem Hadrom and the phloem Leptom, and excluded 

 from his conception of them the mechanical sclerenchy- 

 matous or fibrous tissue often abutting on them. 



Though there is much to be said for such an arrange- 

 ment as this when looked at from the physiological point 

 of view, showing as it does no doubt the meaning and 

 importance of many structural peculiarities as adaptations 

 to environment or as ministering in some way to need, 

 it clearly cannot replace a truly morphological conception. 

 It throws no light on the problems of relationship or 

 descent, for physiological modifications frequently affect 

 very different tissues for identical purposes. Function, 

 bearing a definite relation to environment and serving the 

 plant under a particular condition, perhaps permanent, 



