18 VEGETABLE PHYSIOLOGY. 



must always exist between plants may be less than on the horizontal 

 plane. Consequently, all the conditions necessary to fertility being 

 assumed as equal, the inclined plane will be capable of supporting 

 a larger number of vegetables having vertical stems than the hori- 

 zontal plane. 



The organization of different parts of plants, so worthy in all 

 respects of exercising the sagacity of physiologists, need not be 

 made a subject of minute research in this place. Generalities suf- 

 fice in our agricultural science. This organization, however com- 

 plex it is in appearance, is probably much more simple than is 

 usually believed : we might perchance find the proof of this sim- 

 plicity in the readiness with which orgins, the most dissimilar in 

 their external forms and so different in their functions, undergo 

 modification and transformation one into another, it might almost be 

 said at the will of the observer. Thus tubers, those tleshy amyla- 

 ceous bodies, which accumulate on the subterranean stems of cer- 

 tain vegetables, such as the potato, give birth to a plant which differs 

 in nothing from that which would arise from the seed of the same 

 vegetable. Certain leaves, — those of the orange, of the ficus elas- 

 tica, &c., will do the same. Woody stems, branches severed from 

 the tree and planted in the ground, produce roots and become inde- 

 pendent plants. If the branches of certain shrubs be buried, and 

 their roots be exposed to the air, these last lue soon seen covered 

 with buds and leaves ; while the buried branches acquire a fibrous 

 capillary structure, and in no great length of time they both present 

 the appearance and exercise tiie functions of roots. This singular 

 mutation readily succeeds with the willow, and it was upon this j)lant 

 that the English vegetable physiologist, Woodward, effected it for 

 the first time.* 



The intimate structure of the roots, trunk, and branches, present 

 considerable similarity. Divided perpendicularly to their lonijitudinal 

 axis, three different zones, so dissimilar that it is impossible to con- 

 found them, are discovered in the different concentric layers whicli 

 make up their mass; these are the bark, the wood, and the pith. 

 A more careful examination shows that each of these zones may be 

 further subdivided. 



The exterior of the bark is covered by an extremely thin, nearly 

 transparent and porous pellicle, formed by an asjicmblajje of little 

 adherent scales; this is the cuticle, or epidermis, which encloses the 

 entire vegetable. As it is extensible within certain narrow limits 

 only, it gives way and cracks in prop<irtion as the l)ody of the tree 

 increases in size. The pores of the e[)idermis are minute openings 

 or mouths which communicate with the exterior by an oval orifice, 

 surrounded by a kind of contractile margin. It has been remarked 

 that moisture tends to close these pores, and that drought and the 

 action of solar light tend on the contrary to make them open. The 

 chemical nature of the cuticle which covers the bark appears to in- 

 dicate that it is destined to defend the plant against tlie too direct 



♦ Davy'i .'\griculiunil Chemistry 



