LECTURES ON BOTANY. 



235 



and increase for long and indefinite periods 

 when deprived of root, and apart from the 

 eartli in whicli lliey naturally vegetate. A 

 socotrine aioe has been suspended in a cham- 

 ber of my dwelling-house for upward of seven 

 years, and daring that time has increased 

 gi'eatly in size, weight, and number of leaves; 

 of which latter organs three or lour are de- 

 veloped every year. It is still growing, and 

 as healthy as one of the same kind planted in 

 a pot and kept in a green-house, although its 

 sole source of nutriment must be absorption 

 from the surrounding air through the medium 

 of its leaves. This is no isolated case, but 

 one among many, of a plant growing without 

 roots or perceptible cause of development, 

 simply cited because it has been constantly 

 under my own observation. From it we may 

 nndersland how possible it is that a vegetable 

 rooted in the soil may still derive much of its 

 subsistence from a different source ; nay. that 

 the root may rather even be necessary to se- 

 cure it a fi.xed habitat than for its support. 



The mode in which the root acts where its 

 assimilating functions are required, and the 

 laws of its development, are circumstances in 

 general little appreciated by those whose 

 daUy pursuits are intimately connected with 

 this part of the vegetable economy. The ca- 

 pability of imbibing from the soil seems to be 

 chiefly confined to the extreme ends of the 

 fibres, and hence arises the drooping of plants 

 when they are shifted from one spot to an- 

 other. How carefully soever the transplant- 

 ation may be made, the absorbing extremities 

 are broken off in consequence of their adhe- 

 sion to the soil ; or, otherwise, exposure to 

 the dry atmosphere contracts their tender tis- 

 sue, and renders it for a time, even when re- 

 placed iu the moist earth, incapable of draw- 

 ing the necessary supplies. 



A section or slice of one of the fibres above 

 alluded to, viewed under the microscope, 

 shows it to consist of a coating of very lax 

 and distended cellular tissue, including a cen- 

 tral column of woody tubes and ducts, the 

 outer portion of the tissue having its cells gen- 

 erally much more compressed than those of 

 the interior, somewhat in the man.ner repre- 

 sented in one of the figiu'es of cellular tissue 

 portrayed in the last Lecture ; but the ex- 

 treme points of the root-fibres never exhibit 

 such contraction of the external vesicles, 

 which appears to take place gradually, as the 

 result of age and the occasional defalcation of 

 moisture iu the soil ; to compensate for which 

 change the fibres are continually extending 

 in length during the growth of the plant, and 

 thus peri)etiially present a newly-fonned siu- 

 face of cellular tissue for the absorption of the 

 stimulating and necessary moisture. These 

 extremities are generally thicker than the 

 part of the fibre immediately above them ; 

 and, being of a soft, sponge-like texture on 

 the outside, have received the name of " spon- 

 gioles." It is by not attending to tin preser- 

 vation of the spongioles, or rather from the 

 total ignorance of many practical gardeners 



(475) 



and foresters in regard to their existence, that 

 so many trees and shrubs ai-e killed by being 

 transplanted. The spougiole is the gi-owing 

 point as well as the organ of absorption ; and, 

 owing to the little care talven in removing the 

 plant, or rather in consequence of the utter 

 carelessness with which it is wrenched I'rom 

 the imperfectly loosened soil, by far the great- 

 er number of these little, but important, parts 

 are broken off and left in the ground. Hence 

 probably — nay, we might say certainly — the 

 reason why large trees cannot be successfully 

 transplanted. Growing vigorously, such a 

 tree quickly absorbs all the nutritive matter 

 from the soil in its immediate vic-inity, and 

 extends its roots iiom time to time deeper or 

 more widely distant, to seek the required 

 supply elsevvhere, until the growing points of 

 their intricate ramifications are too far removed 

 from the site of its trunk to be preserved dur- 

 ing the process of shifting. Obvious, howev- 

 er, as are the absorbent functions of the root, 

 as exemplified by these phenomena, no one 

 who has closely studied the varied structure 

 and physiology of vegetables, and marked the 

 diversified habits and modes of growth that 

 often characterize individuals belonging eveu 

 to the same natural family, will maintain the 

 popular opinion of its being tlie universal 

 source of aliment. Among those plants whose 

 vital action cannot be supported without it, 

 even but for a short period, there are various 

 grades in the essentiality of its action as an 

 absorbent organ. This is especially evinced 

 by the diiferent situations in which they veg- 

 etate, or are capable of vegetating. 



It is not intended by any of the foregoing 

 remarks to deny the important agency of the 

 organ under consideration, but only to direct 

 attention to the fact that such agency may be 

 suspended under peculiar circumstances in 

 certain plants : which, still continuing to live 

 and grow, prove that other organs of assimi- 

 lation exist, and, existing, are more or less 

 concerned in the maintenance of vegetable 

 life. The most essential of these latter are 

 the leaves, of which only a very small num- 

 ber among the higher orders of plants are des- 

 titute ; and eveu these tend by tlieir peculiar- 

 ity of habit to throw a valuable light on their 

 physiological influence. 



In regard to the actual functions of the 

 leaves, opinion has been much divided, but 

 they are now very generally considered to be 

 the lungs or breathing organs of the plant ; at 

 die same time it is probable that this fimction 

 is associated with some others, either imifonn- 

 ly acting or dependent on the influence of ex- 

 ternal circumstances, as the presence or ab- 

 sence of solar light, the condition or changes 

 of the atmosphere, &-c. ; and. likcwLse, that 

 in many instances no inconsiderable propor- 

 tion of substance is conveyed into the gtx)w- 

 iug plant through their medium. 



The anatomical structure of a leaf, and the 

 connection which exists between it and the 

 vessels or veins of the wood and inner surface 

 of the bark, are all calculated to maiutaiji tlio 



