PHYSIOLOGY. 



271 



er intimately it may be connected with it. In other terms, 

 the specific action necessary for perfecting the germ, cannot 

 be continued through a succession of layers, when it has not 

 originated in the first of the succession. 



Thus in the section of a trunk of willow, Fig. 240, which 

 consists of two branches, which Fi s- 24 - 



have united into one, the bud a, 

 was found to have moved forward 

 in a direct line from the centre to 

 the circumference ; but 6, another 

 bud, having proceeded as far as c, 

 where the two limbs forming the 

 trunk meet, turns aside, and instead 

 of protruding through the surface 

 titf, in the line of its first direc- 

 tion, appears at b. In this section, d marks the last indepen- 

 dent belt of the branch, and e, that of the main trunk ; the 

 other zones being common to both. 



STRUCTURE OF THE LEAVES. 



The structure of laves is chiefly cellular, the cells pre- 

 senting a variety of forms, but are most commonly globular, 

 or oval, or hexagonal. 



Every leaf has a cuticular covering, which, as we have 

 already seen, is composed of two membranes, the epidermis, 

 and cutis. The epidermis on the leaves is exceedingly thin 

 and delicate, and enters into every pore of the cutis, to 

 which it firmly adheres. The cutis consists of a vascular 

 net-work, which lies upon a layer of air cells. 



The cutis with its lining of epidermis, presents, when ex- 

 amined by a powerful microscope, very curious and inter- 

 esting appearances, being found to consist of pores, or ori- 

 fices placed in various directions with respect to each other, 

 and which communicate by means of open lines, forming a 

 kind of net- work, which differs greatly in form, in different 

 leaves. These pores, sometimes called Stomata, are as 

 above stated, of different forms, but most commonly oval, or 

 hexagonal. The lines, or vessels, by which they are joined, 

 are generally undulated, or zig-zag, and communicating with 

 each other in various directions, as well as with the pores. 

 But figures only, and not words, can convey any idea of the 

 forms of these meshes. 



In plants with parallel ribs, as in Spiderwort, (Trades- 



