STORAGE-TRACHEIDES 407 



a clavate or almost spherical form (e.g. in Euphorbia biglandulosa, 

 E. splendens, E. Myrsinitcs, while in other cases, as Heinricher has 

 shown (Capparis spinosa, C. aegyptiaca, etc., Ccntaurea glomcrata), they 

 represent modified parenchymatous elements pertaining to the paren- 

 chymatous bundle- sheaths, or even to the green mesophyll. At a 

 more advanced stage of specialisation, the modification in question is 

 not confined to the bundle-ends, but affects the whole extent of some 

 of the minor vascular strands. In the leaves of Astrolobium repandum, 

 for example, according to Heinricher, individual cells or cell-groups in 

 the parenchymatous bundle-sheaths are transformed into storage- 

 tracheides, the walls of which are furnished with transversely elongated 



Fig. 160. 



Bundle-ends with storage-tracheides in the leaf of Euphorbia splendens ; m, portion 



of a latex-tube. 



pits ; the modified cells occur principally in the portion of the sheath 

 adjoining the hadrome, or, in other words, in immediate proximity to 

 the conducting tracheides. A similar state of things prevails in certain 

 species of Ccntaurea (C. scoparia, C. americana, C. senegalensis). As 

 regards the general distribution of storage-tracheides of this type, in 

 leaves, it may be stated that they tend to occur more particularly in 

 connection with the marginal veins. 



Many plants that grow in dry, sunny situations are provided with 

 storage-tracheides which have no immediate connection with the 

 vascular system ; the cells in question may be purely idioblastic in 

 character, but in some cases form continuous layers, interpolated 

 between the photosynthetic tissue and the epidermis. This latter 

 condition is exemplified by species of Plcurothallis and Physosvphon. 

 Examination of a transverse section through the leaf of Physosiphon 

 Landsbergii (Fig. 161) reveals the presence of a typical water- tissue, 

 situated beneath the adaxial epidermis, and comprising two layers of 



