SECT. I 



MORPHOLOGY 



127 



and large bordered pits (Fig. 143, t), and often also spirally thickened tracheids 

 ■which serve as water-carriers ; vascular tracheids (gt), with similar functions, 

 but with the structure and thieken- 



AT 



ings of vessels ; fibre tracheids 

 (ft), with small lumina and pointed 

 ends, having only small, obliquely 

 elongated bordered pits, and, in ex- 

 treme cases, exercising merely me- 

 chanical functions ; and finally tra- 

 cheae (g), formed by cell fusion, and 

 provided with all the different forms 

 of thickenings by which they are 

 distinguished as annular, spiral, 

 reticulate, or pitted vessels. All 

 vessels function as water-carriers. If 

 they have small lumina and resemble 

 tracheids, they may be distinguished 

 as tracheidal vessels (tg); if, as is 

 generally the case, they have bordered 

 pits on their lateral walls, they are 

 usually provided with tertiary thick- 

 ening layers in the form of thin spiral 

 bands (Fig. 14S, m). In the paren- 

 chymatous tissue of the wood, the 

 cells (Kg. 144) generally retain their 

 living contents, and never develop 

 true bordered pits with a torus in 

 the closing membrane, which are 

 so characteristic of the water-con- 

 ducting elements. All tissues of this class may be best derived from wood 

 parenchyma. The wood parenchyma is produced by transverse divisions of 

 the cambium cells, and accordingly consists of rows of cells (ftp) with transverse 

 division walls, and others obliquely disposed, which correspond to the alternately 

 differently pointed ends of the cambium mother cells. The cells of the wood 

 parenchyma are provided with simple round or elliptical pits, varying in size in 

 different kinds of wood ; they generally contain starch ; and some of them also 

 take up by-products, resulting from metabolism, or from the chemical changes taking 

 place within a plant in the processes of its nutrition and growth. The cells having 

 the closest resemblance to those of typical wood parenchyma are the so-called 

 fibrous cells (ef). In their contents, as well as in their wall thickenings, they 

 are similar to the cells of wood parenchyma, but are formed directly from one 

 entire cambium cell. In their formation, the cells of the cambium tissue do not 

 undergo a transverse division, but become more or less elongated and fibrous. The 

 libriform fibres or wood fibres (ft) have a similar origin, but are even more elon- 

 gated and have thicker walls, and, at the same time, narrow, obliquely elongated, 

 simple pits. Although the wood fibres may continue living, in the more extremely 

 developed forms (A) they lose their living contents. They are then filled with 

 air, and their function is merely mechanical. Under certain conditions, by later 

 transverse divisions, the libriform fibres may become transformed into septate 

 wood fibres (gh). The transverse septa thus formed remain thin, and form 

 a striking contrast to the more strongly thickened lateral walls. 



Fig. 140. — Part of a transverse section of the stem 

 of a Pine, s, Late wood ; c, cambium ; v, sieve- 

 tubes ; p, bast parenchyma ; k, cell of bast 

 parenchyma, containing crystal; cv, sieve-tubes, 

 compressed and functionless ; m, medullary ray. 

 (X 240.) 



