60 



BIOLOGY OF PLANTS. 



3. The stem and branches are the frame-work or skeleton, for 

 the support of the parts which are necessary to the processes 

 of nutrition. 



2. The roots, in connection with the leaves, serve the pur- 

 poses of mouth and stomach, absorbing and digesting those sub- 

 stances, which are held in solution l)y water or air. 



3. The common vessels are tubes, answering to the lacteals and 

 veins of animals. These tubes pass upward from the root 

 through the stem, and are distributed in minute ramifications, 

 over the surface of the leaves. Through these tubes the sap 

 or circulating fluid ascends. 



4. The leaves are the lungs which perform the office of absorb- 

 ing and exhaling carbonic acid, oxygen, anunonia and water, by 

 which the sap is prepared for its descent and assimilation. 



5. The proper vessels are tubes corresponding to the arteries of 

 animals, extending from the leaves through the inner layer of 

 the bark, to the roots. In these tubes, the prepared nutriment 

 descends, yielding, or forming in its progress, the peculiar sub- 

 stances which belong to the vegetable kingdom. 



6. Finally. " The size of a plant is proportioned to the surface 

 of the organs which are destined to convey food to it." That is, 

 a plant obtains another mouth and stomach with every new fibre 

 of root, and every new leaf; hence, the size depends upon the 

 amount of the leaves and roots. If the leaves be plucked off, 

 the plant will either die, or become stinted in growth. If the 

 roots are diminished, a similar effect will be produced. It is 

 on this principle, that oaks are reared by Chinese gardeners, 

 both in Amsterdam and London, only a foot and a half high, 

 " although their trunks, bark, leaves, branches and whole habi- 

 tus evince a venerable age." As the leaves and roots are per- 

 mitted to increase, they absorb a greater quantity of jiourish- 

 ment. This is not returned to the soil, but is employed in 

 forming new organs. 



The conditions required for the most vigorous action of 

 the vital principle, during the growth of plants, embrace 

 nearly the whole science of agriculture. But I shall confine 

 myself, in this place, to three conditions ; a proper medium 

 and space in which to grow, proper food, and proper tillage. 

 A general view only of these conditions can be given in this 

 connection, a more particular consideration of them will be 

 reserved for future sections. 



