328 THE MICROSCOPE. 



tral miss of the turnip, where the vascular system is relatively imperfect 

 Besides the comparatively regular forms of these absorbents, there are forms 

 composed of amorphous masses of f enestrated cells. It should be added that 

 both the regular and irregular kinds are very variable in their numbers : in 

 some turnips they are abundant, and in others scarcely to be found. Possibly 

 their presence depends on the age of the turnip. 



6. Represents a much more massive absorbent from the same leaf, the sur- 

 rounding tissues being omitted. 



7. Similarly represents, without its sheath, an absorbent from the leaf of 

 Clusia flava. 



8. A longitudinal section through the axis of another such organ, showing its 

 annuli of reticulated cells when cut through. The cellular tissue which fills 

 the interior is supposed to be removed. 



stance of the wood, but to the permeability of its ducts 

 and such of its pitted cells as are united into regular 

 canals ; and the facts showing this at the same time in- 

 dicate with tolerable clearness the process by which wood 

 is formed, for what in these cases is seen to take place 

 with dye may be fairly presumed to take place with sap." 



Taking it, then, as a fact that the vessels and ducts are 

 the channels through which the sap is distributed, the 

 varying permeability of their walls, and consequent for- 

 mation of wood, is due to the exposure of the plant 

 to intermittent mechanical strains, actual or potential, or 

 both, in this way. "If a trunk, a bough, shoot, or a 

 petule is bent by a gust of wind, the substance of its 

 convex side is subject to longitudinal tension, the substance 

 of its concave side being at the same time compressed. 

 This is the primary mechanical effect. The secondary is 

 when the tissues of the convex side are stretched, they 

 also produce lateral compression of them. In short, that 

 " the formation of wood is due to intermittent transverse 

 strains, such as are produced in the aerial parts of upright 

 plants by the action of the wind." Thus the subject is 

 most ingeniously worked out, and the results of many 

 very interesting and instructive experiments are recorded 

 by the author of the paper. 



"In the course of experiments on the absorption of 

 dyes by leaves, it happened that, in making sections 

 parallel to the plane of a leaf, with the view of separating 

 its middle layer, containing the vessels, I came upon some 

 structures that were new to me. These structures, where 

 they are present, form the terminations of the vascular 

 system. They are masses of irregular and imperfectly 

 united fibrous cells, such as those out of v.'hich vessels 



