368 CONDUCTING SYSTEM 



average transpiratory activity of the plant, which in its turn is 

 determined by the nature of the environment. Thus the tissue in 

 question is poorly represented in the Spruce (Picea) and the Fir 

 (Abies), which both prefer a somewhat moist and shady habitat, but 

 is strongly developed in the sun-loving Pine (Pimis). 



According to Strasburger, special " albuminous cells " distinguished 

 by the size of their nuclei and by their large content of protein-material 

 are closely associated with the leptome-strands and form a counter- 

 part to the border of tracheides. The elements in question, which 

 evidently take the place of the transitional-cells of Dicotyledonous 

 bundle-ends and doubtless perform similar functions, have been observed 

 by Strasburger on the flanks of the foliar bundles in a number of 

 Conifers (Pinus sylvestris, Abies pectinata, Picea excelsa, Juniperus com- 

 munis, Taxus baccnta) and in Cycas circinalis ; they can only be clearly 

 made out in spirit material. 



D. THE ENDODERMIS (SCHUTZSCHEIDE). 



In a great many plants the vascular tracts are separated from the 

 adjoining tissues by special sheathing layers, which sometimes surround 

 each individual conducting strand, and in other cases extend continu- 

 ously around the entire vascular cylinder. This layer, which does not 

 possess the same morphological value in every case, was termed the 

 protective sheath (tichutzschr i<h) by Caspary, but in English text-books is 

 more generally known as the endodermis. As a rule endodermal layers 

 differ markedly in their anatomico-physiological characters from carbo- 

 hydrate-conducting parenchymatous sheaths. Occasionally, however, 

 a sheath of the latter type undertakes the duties of an endodermis as a 

 subsidiary function, in which case it also acquires some of the correla- 

 tive histological features ; according to Schwendener, this condition is 

 exemplified by the leaves of those Grasses which do not possess a 

 typical endodermis. 



Endodermal layers serve, in the first instance, to restrict transloca- 

 tion to certain definite paths, and to prevent the premature escape of 

 substances which are travelling in the vascular bundles. In addition 

 they often afford mechanical protection to the conducting strands. 

 With these two physiological functions are correlated two outstanding 

 structural features of the endodermis, namely its relative irnperviousness 

 and its mechanical strength. 



The cells of the endodermis are always in uninterrupted contact 

 with one another. They are usually of an elongated parenchymatous 

 form, but in exceptional cases become distinctly prosenchymatous. 

 Frequently all the walls are suberised, in which case, according to Von 

 Hohnel and Kroemer, they resemble the membranes of cork cells, the 



