136 



BOTANY 



PART I 



\-t 



,v 



of the water-conducting 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 (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-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 resem- 

 blance 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 each [is formed directly from one entire 

 cambium cell. In' their formation, the cells of 

 the cambium tissue become more or less elong- 

 ated and fibrous. The LIBRIFORM FIBRES or 

 WOOD FIBRES (h) have a similar origin, but are 

 even more elongated and have thicker walls, 

 and, at the same time, narrow, obliquely elong- 

 ated, simple pits. Although the wood fibres 

 may continue living, in the more extremely 

 developed forms (h) 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 trans- 

 verse septa thus formed remain thin, and form 

 a striking contrast to the more strongly thick- 

 ened lateral walls. \ While the tracheal tissues 

 are engaged in providing for the conduction of 

 water, the duty of conducting and storing the 



pit; tm, tracheidal medullary ray products of assimilation, in particular the 

 cells; am, medullary ray cells con- carbohydrates, is performed by the parenchy- 

 ' matous tissues of the wood. Both forms of 



tissue, however, aid in maintaining the rigidity 

 of the plant body, and, in their most extreme 

 development, furnish such elements as the fibre tracheides on the one hand, and 

 on the other the empty wood fibres, which are only capable of performing 

 mechanical functions. 



The wood of Dicotyledons is made up of the elements of these two classes of 

 tissue, the tracheal and the parenchymatous, but all the different elements are not 

 necessarily represented in any one kind of wood. 



Drimys, a genus belonging to the Magnliaceae and two genera of the related 

 order Trochodeiidraceae, are the only Dicotyledons the wood of which is formed of 

 tracheides only. These Dicotyledons closely resemble the Conifers in structure. 

 In numerous Leguminosae, Willows, Poplars, and species of Ficus, on the other 

 hand, the tracheal tissues are only represented by vessels, which perform the task 



FIG. 144. Tangential section <>t the 

 autunm-wood of a Pine, t, Bordered 



on one side; i, intercellular SJKII-I- in 

 the medullary ray. (X 240.) 



