Vegetable Physiology. 



381 



waxy excretions, and the impregnation of the membrane with 

 silica, will be spoken of more conveniently hereafter. 



Fin 



■,[V]llll 





L -4_~ J 



Perpendicular section of the outer part of a grain of linseed : a, the cuticular pellicle ; &, the epidermal 

 cells filled up by laminated deposits of the substance which dissolves into a jelly ; c, subjacent 

 layer of cells of the seed-coat partly obliterated by pressure ; d, layer of cells containing 

 albuminous substance ; e, cells of the endosperm, containing drops of oil and granular proto- 

 plasm. Magnified 350 diameters. 



In the lower Cellular plants the fluids conveying nutriment 

 appear to be diffused with great uniformity throughout the whole 

 structure. Tn plants possessing a stem, and in particular in the 

 Flowering plants, with which we are more especially concerned, 

 the fluids absorbed by the roots flow up in determinate courses 

 through tracts of a tissue which differs in important respects 

 from the parenchyma above described. The cells lying in these 

 tracts exhibit at a very early period a peculiarity of form ; they 

 become elongated, and more tubular than sac-like; and if examined 

 in young shoots or stems they will be found to contain colourless 

 matters, while the surrounding parenchyma abounds in chloro- 

 phyll, &c. At a very early period of their existence, moreover, 

 the character of a portion of these cells becomes very distinctly 

 marked by the alterations which take place in the cell-wall. 

 Secondary deposits or layers of thickening are formed upon tlie 

 walls, in lines which have a more or less distinctly spiral course, 

 so that the new substances form spiral bands or ridges upon the 

 inside of the tubular cell, visible externally through the trans- 

 parent wall ; very often this thickening occurs, especially in the 

 larger tubes, in the shape of rings, or of rings or spiral coils con- 

 nected into a kind of net- work by cross pieces (fig. 5). Cells of 

 this character, more or less elongated, occur in the ends of young 

 shoots of ordinary trees, forming the rudiments of the wood, ex- 

 tending out in bundles into the leaves and other organs as the prin- 

 cipal constituent in the ribs and veins, which indeed, in the parts 

 of the flower, are chiefly composed of these ' spiral vessels.' The 

 slender threads traversing the spongy substance of the leaf-stalk 



