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A TEXTBOOK OF THEORETICAL BOTANY 



a mechanical, strengthening tissue. It has this great advantage over 

 sclerenchyma, that its cells are living and growing and hence it is able to 

 adapt itself to the growth of young organs. It is chiefly characteristic of such 

 organs, nevertheless in short-lived structures such as petioles or the herbaceous 

 stems of Dicotyledons it may remain as the permanent mechanical tissue. 

 The aerial stems of Monocotyledons, on the other hand, depend for mechanical 

 support more on the development of the sclerotic sheaths around their 

 s^ascular bundles than on collenchyma. 



As collenchyma cells remain alive it follows that their cell walls cannot be 

 uniformly thickened, for this would cut them off from all external supplies. 

 The thickening material, which is of cellulose, is accordingly distributed in 



Sclerenchyma 

 Assimilating tissue 



Xylem 



Pith 



Fig. 858. — Sarothamnus scoparius. Transverse section of 

 stem showing ridges with sclerenchyma, and assimilat- 

 ing tissue in the cortex. 



such a way that thin walls are left at certain parts of the cell to allow of inter- 

 communication. Frequently the thickening appears only at the angles of the 

 cells, while in other cases it may be all on one side or on two sides, an arrange- 

 ment which also has the advantage of increasing the plastic and yielding 

 character of the tissue as a whole. The cells are somewhat elongated 

 vertically, but never to the extent of woody fibres, and they may be regarded 

 as mainly parenchymatous. 



Cortical cells usually contain functional chloroplasts since light can 

 penetrate a distance up to 100 m from the surface. True palisade tissue, 

 comparable with that in the leaf, may be found in the cortex of some xero- 

 phytes, either in succulents of the Cactus type or in switch plants, like the 

 Broom (Fig. 858), in fact, wherever the stem has taken over the function 

 of photosynthesis from the leaves, either owing to the reduction of the latter 

 to mere scales or to their complete disappearance. 



Where chlorenchyma, i.e., chloroplast-containing tissue, is present, 



