146 



BOTANY 



PART I 



early stage, and in their fully developed condition are in reality only dead cell 

 cavities. In this class are included tracheides having relatively wide lumina and 

 large bordered pits, and ultimately also spirally thickened walls, which serve as water- 

 carriers (Fig. 154 A, t) ; vascular tracheides {gt), with similar functions, but with 

 the structure and thickening of vessels ; fibre tracheides {ft), with small lumina 

 and pointed ends, having only small, obliquely elongated bordered pits, and, in 



extreme cases, exercising merely mechanical 

 functions ; and finally trachea {g), formed 

 by cell fusion, and provided with all the differ- 

 ent 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 re- 

 semble tracheides, 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 

 thickening layers in the form of thin, spiral 

 bands (Fig. 158 m). In the parenchymatous 

 tissue of the wood the cells (Fig. 154 B) gener- 

 ally retain their living contents, and never 

 develop the true bordered pits with a torus in 

 the closing membrane, which are so character- 

 istic of the water-conducting elements. All 

 tissues of this class may be best derived from 

 wood parenchjniia. The wood parenchyma is 

 produced by transverse divisions of the cambium 

 cells, and accordingly consists of rows of cells 

 {hp) 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 bye-juoducts, result- 

 ing from metabolism, i.e. from the chemical 

 changes taking place within a plant in the pro- 

 cesses of its nutrition and growth. The cells 

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

 FIBROUS cells {cf). In their contents as well as in their wall thickenings, they 

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

 one entire cambium cell. In their formation, the cells of the cambium become 

 more or less elongated and fibrous. The libriform fibres or wood fibres (/() 

 have a similar origin, but are even more elongated and have tliicker walls, and, at 

 the same time, narrow obliquely elongated, simple pits. Although the wood fibres 

 may continue living, they lose their living contents in the more extremely 

 developed forms (/t). 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 



Fig. 153. — Tangential section of tlie au- 

 tumn wood of a Pine, t, Bordered 

 pit ; tm, tracheidal medullary ray cells; 

 sm, medullary ray cells containing 

 starch ; et, pit bordered only on one 

 side ; i, intercellular space in the 

 medullarj- ray. (x 240.) 



