38 



THE ELEMENTARY STRUCTURE OF PLANTS. 



not unfrequently distinguishable, when highly magnified, into more 

 or less defined concentric layers ; as shown in Fig. 18, from the 



inner bark of the Birch, and 

 in Fig. 19, in some cells of 

 proper wood. Whether the 

 thickening deposit is distin- 

 guishable into layers or not, 

 it is more commonly inter- 

 rupted at certain points and 

 in a definite way, so as to 

 give the diminished cavity 

 very irregular outlines ; as 

 we see in Fig. 16 and Fig. 

 17. This occurs in wood-cells as well as in ordinary rounded 

 cells, and is partly shown in Fig. 19. The earliest layers of thick- 

 ening fail to be deposited at certain points, consequently leaving 

 thinner spots ; the succeeding layers are exactly applied to the 

 next preceding, and leave precisely the same intervals : conse- 

 quently, these unthickened spots become grooves or canals running 

 from the cavity of the cell to the original wall, or in that direction. 

 And it is noticeable that the pits or canals of contiguous cells usu- 

 ally correspond : an obvious effect or use of this adaptation is to 

 maintain a lateral communication between contiguous cells of the 

 kind, notwithstanding the thickening of their walls. No tissue 

 which we have seen shows these lateral passages and their nature 

 more clearly than the wood of the American Plane-tree, or Button- 

 wood (Fig. 22), which at the same time demonstrates the true 

 character of one large class of the 



43. Markings Of the walls Of Cells, These, whether in the form of 

 bands, spiral lines, dots, or apparent pores, all arise from the un- 

 equal distribution of the secondary deposit. They are portions of 

 the walls which are either thinner or thicker than the rest. These 

 markings display the greatest variety of forms, many of them of 

 surpassing elegance. The principal kinds occur with perfect uni- 

 formity in each species or family, and in definite parts of the 

 plant ; so that, in a multitude of cases, a given species or genus 



FIG. 18. Highly magnified cross-section of a bit of the old liber of the bark of the Birch ; 

 the tubes nearly filled with a deposit of solid matter in concentric layers. (From Link.) 



Highly magnified wood-cells (seen in transverse and longitudinal section), from 

 the root of the Date Palm ; showing the internal deposit in layers, and some connecting canals 

 or pits. (From Jussieu, after Mirbel.) 



