86 



STRUCTURAL BOTANY 



These differences, however, do not otherwise affect the 

 anatomy. Although the same general structure is 

 maintained throughout the shoot, yet in the minute 

 ultimate branches the number of bundles becomes 

 much reduced, often down to three, and in these cases 



the appearance of 

 sf. the transverse 



section may be 

 very different from 

 that of a main 

 stem or larger 

 branch (see Fig. 

 40). 



We will now 

 consider the tissues 

 rather more in 

 detail, and will 

 begin with the 

 vascular bundles. 

 Each bundle is 



FIG. 40. Equisetum limosum ; transverse normally collateral, 



section of an ultimate branch of the aerial -,-1 i orr> ^-n 



stem, v.b, the three vascular bundles, each l - e - W1 | n X J L 



with its o\vn endodermis ; st, the depressed its inner and 



stomata. The pith is hollow, but there are -, .. 



no vallecular cavities. Almost the whole Phloem On ItS Outer 



cortex is assimilating palisade -tissue. Mag- side. The Cailual 



nified 100 diameters. (R. S.) .. , 



cavity marks the 



position of the protoxylem or first formed tracheides 

 of the bundle (see Fig. 39). Here a few tracheides 

 have become thickened (in an annular or spiral 

 manner) at a very early stage of growth ; consequently 

 they cannot follow the expansion of the surround- 

 ing tissues, and a rupture takes place, forming the 

 cavity. Projecting from the walls of this cavity we see 



St. 



