38 



ORGANOGRAPHY. 



this kind of tissue as much as possible. Parenchyma is con- 

 nected by various intermediate conditions with jprosenchyma, 

 which we now proceed to notice. 



Fig. 69. Fig. 70. Fig. 71. 



Fig. 69. Prosenchymatnus cells. Fig. 



70. Horizontal section of prosenchy- 



matous cells. Pig. 71. Proseucliyma- 



tous cells in coiubination. 



2. Prosenchtma. — The 

 most perfect form of pros- 

 enchyma is that common- 

 ly termed Woody Tissue, 

 {Woody Fibre of the old 

 writers). 



This tissue consists of 

 very fine cells, elongated and 

 tapering to a fine point at 

 each of their extremities, 

 their inside being much 

 thickened by secondary de- 

 posits {fig. 69), and when 

 in contact with each other, 

 overlapping by their pointed 

 ends, so that they are firmly 

 compacted together and leave 

 no interspaces {fig. 71). The 

 woody portions of all plants 

 consist in a great part of this 

 form of tissue. It is also 

 found in the liber or inner bark mixed with parenchyma, and 

 in the veins of leaves and those of other expansions of the stem 

 and its divisions. 



We distinguish three kinds of cells which enter into the com- 

 position of Woody Tissue ; namely, the ordinary Wood-cells, Disc- 

 bearing or Pzmctatcd Wood-cells, and Liber-cells ; these form 

 respectively, by their combination, the ordinary Woody Tissue, 

 the Disc-hearing or Punctated Woody Tissue, and Woody Tissue 

 of the Liber. 



a. Woody Tissue. — This, the ordinary kind of woody tissue, is 

 composed of cells the walls of which, although thickened by 

 secondary layers, either present a homogeneous appearance, as is 

 more commonly the ease, or are marked with little dots or pits, 

 as in pitted cells. The occurrence of spiral fibres, or rings, or 

 reticulations, is exceedingly rare in wood-cells. The secondary 

 deposits are arranged in concentric layers, whicli increase in 

 number as the cells progress in age, so that in old wood their 

 cavities are often nearly obliterated {fig. 70). This kind of tissue 

 occurs in the wood of most trees, except that of the Coniferse 

 and allied orders: and in the veins of some leaves, and those 

 of certain parts of the flower. The peculiar manner in which 

 these wood-cells are arranged with respect to each other, over- 

 lapping at their pointed extremities, and thus becoming firmly 

 cemented, as it were, together, combined with the thickness of 



