THE PLANT-CELL 



57 



inal form, although sometimes much elongated. The whole of the 

 body of the lower plants, and most of the active tissues of the higher 

 ones, are parenchymatous. 



Mechanical Tissues. Plants growing in the air require certain skel- 

 etal structures to give them the necessary rigidity. These support- 

 ing tissues are known as mechanical tissues, but are not necessarily 

 devoted to this purpose only. The strongly distended cells of ordi- 

 nary parenchyma give firmness, and may to some extent be consid- 

 ered mechanical tissue, but large aerial plants require something 



~vb 



FIG. 38. A, cross-section of the stem of a Begonia, showing the circle of vascular 

 bundles (x 3). B, collenchyma, or thick-angled tissue from the outer part of the 

 cortex of the same plant, more highly magnified. C, cross-section of the peduncle 

 of the inflorescence of Phutnix Canariensis, showing the numerous scattered vascu- 

 lar bundles (X 2). D, cross-section of an internode of the shoot of Equisetum 

 lavigatiim, showing the ring of vascular bundles, vb, alternating with large air- 

 spaces, 1. E, sclerenchyma, or fibrous tissue, from the outer part of the stem 

 (X 250). The shaded portions of A and C indicate the mechanical tissues. 



more, and we find special tissues developed. In the vascular plants 

 there is generally found below the epidermis a greater or less devel- 

 oped system of supporting tissues (Hypoderma), which may be in 

 the form of elongated, thick-walled fibres, with pointed ends (Prosen- 

 chyma, e.g. Wood-fibres), or thick-angled elongated elements (Collen- 

 chyma, e.g. Begonia), or shorter, very thick-walled stony cells 

 (Sclerenchyma, e.g. the rhizomes of most Ferns). 



Most important in this connection are the vascular bundles of the 

 higher plants, which form a very complete skeleton of firm, woody 



