88o A TEXTBOOK OF THEORETICAL BOTANY 



Fig. 866. — Ribes aureum. Transverse section 

 of an injured stem showing above, a layer 

 of wound cork and below, a parenchyma- 

 tous callus with fragments of pericyclic 

 sclerenchyma. {After Kuster.) 



features, e.g., in some Compositae, species of Primula, and in the rhizomes 

 of some Monocotyledons (Fig. 867). The endodermis, in most cases 

 where it occurs in the stem, is a common investment round the stele as 

 a whole, but there are a number of instances among Angiosperms where 

 each vascular bundle has an individual endodermis (Fig. 868), which is not 

 infrequently continued into the leaves {Adoxa, Menyanthes, and many Grasses, 

 such as Feshica). Species even within the same genus may differ in this 

 character as, for example, in Ranunculus, where R. lingua and R. flammula 

 are peculiar in having individualized bundle sheaths. 



The significance of the endodermis and of its variations cannot as yet 

 be fully interpreted on physiological grounds, but some interesting points 

 have been elucidated. It first appears outside the procambial zone at the 

 stem apex,* simultaneously with the first appearance of xylem elements. 

 Materials from the breakdown of the xylem protoplasm probably diffuse out 

 radially, and may supply the fatty acids which condense to form the Casparian 

 Band. It has been suggested that the condensation of fatty acids at this level 

 is due to the penetration of air to this zone through the intercellular spaces 

 of the cortex, but while we may have here the foundation for an explanation 

 it is plainly not the whole story. 



* In Juncaceae and Cyperaceae it arises in the procambium. Its status as part of the 

 cortex is not therefore universal. 



