82 



BOTANY 



PART I 



render them stiff and brittle. Silica is also present in the superficial 

 cell walls of the Gramineae, Equisetaceae, and many other plants. 



Cell Forms. — As cytoplasm is a viscous fluid, and would tend, if 

 unimpeded, to take a spherical shape, it may be assumed that the 

 natural and primary form for cells is spherical. Such a shape, how- 

 ever, could only be realised by cells which, in their living condition, 

 were completely free and unconfined, or in such as were able to ex- 

 pand freely in all directions. Newly-developed cells, which are in 

 intimate union, are, at first, always polygonal. Through subsequent 

 growth their shape may change. The cubical cells of the growing 

 point either elongate to a prism or remain short and tabular. If the 



growth is limited to certain definite points, 

 and is regular, they become stellate ; if irre- 

 gular, their outline is correspondingly un- 

 symmetrical. In consequence of energetic 

 growth in length, fibre-like, pointed cells 

 are developed. If the walls of such cells 

 become much thickened, they are called 

 sclerenchyma fibres (Fig. 90, A). These 

 show diagonal markings, due to their elon- 

 gated pits, which are generally but few in 

 number. When fully developed, the living 

 contents of such cells are small in amount and 

 frequently they contain only air. In the last 

 case, they merely act. as mechanical supports 

 for the other parts of the plant. Cells some- 

 what similar, but shorter and considerably 

 wider, not sharpened at the ends, and pro- 

 vided with bordered pits, are called tra- 

 cheids (Fig. 90, ■£). The tracheids, in 

 their fully developed condition, never have 

 any living contents, but serve as water- 

 carriers for the plant. So long as they 

 remain active, they contain only water and 

 isolated air-bubbles ; their active functions 

 afterwards cease, and they become filled 

 with air. Tracheids, which are specially 

 elongated, and at the same time have only 

 a narrow lumen, and, like the sclerenchy- 

 matous fibres, serve merely mechanical 

 purposes, are known as fibre tracheids. 

 Very long tracheids with a wide lumen and thin walls, functioning, 

 like typical tracheids, as water-carriers, are distinguished as vasiform 

 or VASCULAR. TRACHEIDS. They are characterised by the annular, spiral, 

 or reticulate markings of their thickening layers, and may also be 

 provided with bordered pits. 



Fig. 90. — A, A sclerenchymatous 

 fibre ; B t a tracheid ; C, part of 

 a spiral tracheid ; D, part of a 

 latex tube. (A, B, C, x 100 ; D, 

 X circa 150.) 



