222 Practical Plant Biology. 



elements composing them, but also by the reduction in size of the 

 elements themselves. The veins branch repeatedly in the leaf, 

 tapering smaller and smaller, until they disappear altogether. 

 The conducting tracts running in them suffer a similar diminution 

 and finally also lose themselves in the green tissue. At their ex- 

 treme tips the conducting tracts consist of one or two spiral 

 tracheids accompanied by a similar number of narrow elongated 

 cells representing the bast. Throughout the leaf the bast is on 

 the under side of the wood, and this relative position is retained 

 as the conducting tract, now rendered more massive by the coming 

 together of the various branches, passes into the rachis and leaf- 

 stalk. Five or six of these tracts enter the stem from each leaf- 

 stalk, and, after crossing the cortex, turn down the stem just out- 

 side the pith. The largest of the conducting tracts, coming down 

 from the middle of the leaf-stalk, enter among the tracts already in 

 the stem coming from higher leaves. They pass down a consider- 

 able distance in the stem before they merge with other conducting 

 tracts. The lateral tracts of the leaves fuse immediately with the 

 conducting tracts of the stem. 



The green tissue of the leaf of the Buttercup is very distinctly 

 differentiated into an upper stratum of palisade-parenchyma and a 

 lower one of spongy-parenchyma. The cells of the latter are very 

 irregular in form and are loosely put together, being in contact with 

 their neighbours at only a few points. Between them are large 

 intercellular spaces. As is usual, these spaces communicate with 

 the outside through stomata in the lower epidermis. The cells 

 of the palisade layer are comparatively regular in shape, being 

 cylindrical with rounded ends. Between them are spaces which 

 are continuous with those of the spongy-parenchyma. The walls 

 of these two layers of cells composing the mesophyll are thin. 

 The cells are green owing to the large number of chloroplasts in 

 the layer of cytoplasm lining their walls. In this cytoplasm a 

 nucleus is also usually seen. The cells are tense and rigid, owing 

 to the osmotic pressure of the solutes in their vacuoles. The meso- 

 phyll is covered over above and below by the epidermis which 

 resembles the epidermis of the stem. Stomata are for the most 

 part found only in the lower epidermis. The veins containing the 

 conducting tracts are composed of cylindrical cells lying parallel to 

 the direction of the veins. The longitudinal walls of these cells 

 are thick and evidently strong enough to resist a considerable 

 tension. The finest ramification of the conducting tracts are em- 

 bedded in the upper layers of the spongy-parenchyma. 



The roots are developed inside the cortex of the stem or bulb, 



