570 LECTURE LVIII. 



fishes : for plants have need of air, and it has been found, that even seeds 

 will not germinate in a vacuum. As the lungs of animals appear to be 

 concerned in forming the blood, so it may be inferred from Mr. Knight's 

 experiments, that the sap first ascends to the leaves through the external 

 fresh wood of alburnum, and through the central vessels of the young 

 leaves and branches, derived from the alburnum, and accompanied by the 

 spiral tubes ; and after being perfected by exposure to light and air in the 

 leaves, it descends in the bark, and serves for the secretion of the alburnum, 

 and of the internal layers of the bark, being conveyed probably by two 

 distinct sets of vessels. The sap, thus prepared by the leaves in the 

 summer and autumn, is supposed to leave its extractive matter in the tree 

 throughout the winter, in such a state as to be ready to unite with the 

 aqueous juices, which ascend from the root in the succeeding spring. The 

 internal parts of the wood, having served the purposes of vegetation, are har- 

 dened, and perhaps dried up, so as to be afterwards principally subservient 

 to strength alone. By subsequent experiments, Mr. Knight has also found, 

 that when a branch hangs downwards, the sap still appears to proceed 

 from the part of the bark which is uppermost ; so that the direction of the 

 force of gravity seems to be concerned in determining that of the motion of 

 the sap. There appears also to be some reason to suppose that mechanical 

 means assist in the protrusion of the sap, and the consequent growth of the 

 tree ; for if a tree be more agitated by the wind in one direction than in 

 another, its diameter will be greatest in that direction. 



The process of grafting depends on a remarkable property of the growth 

 of vegetables ; if the cut surface of the inner bark of a small branch, or 

 cutting, be placed in contact with that of the branch of another tree, they 

 will unite sufficiently for the nourishment of the cutting ; provided, how- 

 ever, that the nature of the plants be not too different. Something of the 

 same kind occurs in animal life, where a tooth has been transplanted into 

 the socket of another, or where the spur of a cock has been inserted into his 

 comb. 



Plants have their natural periods of life, either of a few days, as in the 

 case of some of the fungi, of a year, of a few years, or of many centuries. 

 They have also their diseases ; they are often infested by insects, as in the 

 gall of the oak, and the woodruff of the rose, or by animalcules of a still 

 lower order, which are either the causes of the smut of corn, or constant 

 attendants on it. From unnatural and too luxuriant culture, they become 

 sterile, and produce double flowers instead of fruits and seeds. When 

 deprived of sufficient moisture, or nipped by frost, their leaves and branches 

 often die ; and if the plants recover their vigour, a separation is effected 

 by a natural process, resembling the sloughing of decayed parts of animals ; 

 but when the whole plant sinks, the dead leaves continue to adhere to it. 

 The annual fall of leaves in autumn appears to be a natural separation 

 nearly of the same kind, which takes place when the leaves are no longer 

 wanted ; the growth of the plant being discontinued, and their functions 

 being no longer required. 



Succulent plants generally die when the cuticle is removed, but not all 



