48 



HISTORY OF THE VKGETABLE KINGDOM. 



they are composed. It remains now to explain 

 the manner in whicli tlie nutritive matters are 

 taken into the plant, and thence circulated and 

 elaborated, so as to become part of the organized 

 structure of the vegetable. In this description, 

 however, we shall confine ourselves at present 

 chiefly to the course of the juices in the sap 

 vessels; as after describing the remaining organs 

 of plants, we shall have to return to the physi- 

 ology of vegetation, properly so called. 



A plant then, ns we have seen, is an organized 

 body, with a variety of structure suited to the 

 various purposes of nutrition. Before it can 

 exhibit the phenomena of life, and grow, and 

 increase in bulk, certain external stimuli are 

 necessary. These are, in the first place, a supply 

 of certain nutritious matters held in solution by 

 water, or in the form of gas or air. The other 

 stimulants are heat, light, and electricity. Water 

 is the necessary vehicle of the nutritious sub- 

 stances of plants ; but it is also decomposed, and 

 its constituents, hydrogen and oxygen, enter into 

 their composition. It is not the sole nutriment 

 of these, however, as was supposed by the older 

 naturalists ; for if a plant be made to vegetate 

 in pure or distilled water, without access to any 

 other substance, it will soon perish. Carbon, 

 silex, lime, soda, potass, the oxides of iron, and 

 some other metals, all enter into the vegetable 

 structure through the medium of the moist soil. 

 The air of the atmosphere also aflx)rds oxygen, 

 both in its simple state and combined with car- 

 bon, forming carbonic acid. Nitrogen, the other 

 ingredient of the atmosphere, also enters in small 

 proportion into their substance. 



Various experiments have been instituted, to 

 show that plants really obtain the various matters 

 of which they are composed, from the soil and 

 atmosphere. Thus, two grains of buckwheat 

 were made to germinate in a little pure sulphur, 

 placed in a platina cup, and moistened with dis- 

 tilled water, over which was put a glass bell, 

 the more carefully to exclude all foreign matters. 

 In the course of a few days the seeds germinated, 

 and at the end of a fortnight had thrown out 

 roots and leaves. The whole was now collected 

 and analysed, and their product exhibited exactly 

 the same proportion of phosphate and carbonate 

 of lime, and silex, that the same weight of other 

 similar seeds contained, which had not been sub- 

 jected to germination. 



Before proceeding to detail the various experi- 

 ments illustrative of the absorption and ascent 

 of the sap in vegetables ; we shall shortly des- 

 cribe the process as it is now supposed to take 

 place according to the most recent investigations ; 

 and for this purpose we shall use the annexed 

 diagram. 



The juices of the moist soil are absorbed by 

 the roots a a, by means of minute spongioles 

 attached to the extremities of these roots. This 



watery fluid having entered by the roots, mingles 

 with the sap already in the stem of the plant, 



ita. 



and mounts upwards by vessels near the central 

 parts of the woody fibre surrounding the pith, 

 as represented by the dotted lines b b ; having 

 traversed the trunk it then enters the branches, 

 and at last reaches the leaves ; here it combines 

 with air absorbed fi-om the atmosphere, through 

 tlie pores or stomata of the leaves, c c. It here 

 also gives off its supei-fluous water, and altogether 

 becomes a different kind of fluid from what it 

 was in its ascent. It now constitutes the proper 

 juice or nutritious fluid of the plant, and again 

 descendsfromtheleaves, through a series of simple 

 tubes in the liber or bark d d, and then is de- 

 posited so as to form new wood bark, and other 

 ]iarts of the plant. Such is the process whicli 

 has been called the circulation of the sap. 



We are indebted to the celebrated natural 

 philosopher Hales, for demonstrating by the most 

 accurate and ingenious experiments the prodi- 

 gious power of suction, of which the roots and 

 branches are possessed. He exposed one of the 

 roots of a pear-tree, cut off its extremity, fitted 

 to it one of the ends of a tube filled with water, 

 having the other end immersed in a mercurial 

 trougli, and in six minutes the mercury rose 

 eight inches in the tube. To measure the force 

 with which the vine absorbs humidity in the 

 ground. Hales made an experiment, the results 

 of which might appear inaccurate and exagger- 

 ated, had they not been verified of late years by 

 Mirbel, who repeated the experiment. The 

 English philosopher, on the 6th of AprO, divided 

 a vine shoot without twigs, of about seven or 

 eight lines in diameter, and at a height of thirty- 

 three inches above the ground. lie then fitted 

 to it a doubly bent tube, which he filled with 

 mercury up to the curve which surmounted the 

 transverse section of the stem. The sap which 



