374 CONDUCTING SYSTEM 



layers in the leaves of Grasses cannot therefore depend upon ecological 

 factors, but must be entirely governed by physiological considerations, 

 such as differences in the methods of translocation, or possibly still unde- 

 tected tissue-correlations, which are not the same in the various tribes of 

 the family. Similarly, the significance of the " individual " and the 

 "common" types of endodermis cannot be explained, until all the possible 

 determining factors both physiological and phylogenetic are properly 

 understood. No reason, for instance, can be given at present for the 

 fact that a " common " endodermis occurs in the stems of Ranunculus 

 aconitifolius, R. pamassifolius, and R. amplexicaulis, whereas R. Lingua 

 and R. Flammula, though belonging to the same section of the genus, are 

 nevertheless provided with protective sheaths of the " individual " type. 



IV. THE ARRANGEMENT AND COURSE OF VASCULAR 



BUNDLES. 17 ' 



The various conducting strands of the plant collectively form a 

 system which traverses every part of the shoot and root. The vascular 

 system may even extend into epidermal appendages, if local translocation 

 within the latter is unusually active for one reason or another ; this 

 latter point is illustrated by the digestive glands of Drosera rottindifolia, 

 which will be dealt with in more detail later on (Ch. X. II. B. 1). Where, 

 on the other hand, translocation is reduced to a minimum, the vascular 

 bundles may be entirely suppressed : this extreme reduction is exempli- 

 fied by Woljjla, and by a few other small floating plants belonging to 

 the same family (Lemnaceae). Evidently physiological requirements have 

 a large share in determining the development of the conducting system. 



The disposition of the vascular bundles in the different parts of the 

 plant-body, displays a variety which is scarcely inferior to the diversity 

 of arrangement with which we have already become acquainted in the 

 case of the mechanical system. It is, however, much more difficult, in 

 the present instance, to correlate the arrangement in every case with the 

 underlying physiological principles. As has already been explained, an 

 ordinary vascular bundle represents a combination of heterogeneous 

 tissues. Hence that particular course of a bundle which is the most 

 advantageous, so far as the protein-conducting leptome is concerned, is 

 not necessarily best suited to the requirements of the water-conducting 

 tissue. Moreover, the disposition of the vascular bundle is determined 

 by mechanical requirements as well as by considerations of nutrition. 

 Thus, delicate conducting strands tend to seek the shelter of skeletal 

 elements, while, on the other hand, stout resistant vascular bundles can 

 themselves extend protection to more vulnerable tissues. The external 

 morphological features of the plant-body, finally, also influence the 



