VEGETABLE PHYSIOLOGY— THE ROOT. 



nected with the alburnum. The accumulation of cells is not upon its sides, but at the ex 

 tremity under the thin epidermis and the superficial cells. The division of the cells from 

 this point proceeds from below onwards, those which lie behind quickly extending to their 

 full size, and remain in that state, while those which approach the apex multiply by con- 

 tinual divisions. In this way the root keeps on growing, and may be compared to an 

 icicle, which lengthens from the point only; the only real difference being that the icicle 

 elongates by continual accretions from the outside, while the growth of the root is from 

 the inside. As this growth of the root is made fi'om the under side of the cxtremitj' alone, 

 it follows that that part is always clothed with a vitally active tissue. " The new 

 cells,* however, do not occupy the extremity alone, as is commonly but incorrectly stat- 

 ed; this is capped, as it were, by an obtusely conical mass of older cells, consisting of 

 the superficial tissue of the end of the radicle, pushed forward by the cell multiplication 

 that commences behind it, as already mentioned. 



" As the orignal cells of this apex wear away or perish, they are replaced by a layer beneath, 

 and so the advancing point of the root, consisting, as inspection plainly shows, of older 

 and denser tissue than that behind it, (the point of every branch of the root is capped in 

 this way,) it follows that the so called spongioles, or spongclds, have no existence. Not 

 onl}'' are there no such organs as are commonly spoken of, but absorption does not 

 evidently take place to any considerable extent, through the older tissue of the point itself." 

 Roots absorb nourishment by endosniosis throughout the whole of the newly formed tis- 

 sue, and especially through the hair-like prolongations, commonly called the fibrils; these 

 capillary tubes are of great tenuity, and have extremely delicate walls, and perform a 

 more important part in absorption than is generally supposed. They perish soon as the 

 growing season is ended, or when the roots become old and hardened — " at the same time 

 the external layer of cells that bears them, at first indistinguishable from the parenchy- 

 ma beneath, except perhaps in the size of the cells — hardens and thickens into a kind of 

 epidermis, or firmer skin, so as to arrest or greatly restrain the imbibition. This epider- 

 mis of the root consists of less compressed cells than in other parts exposed to the light, 

 and is distributed to stomates, or breathing fibres." The growth of the root keeps pace 

 with the stem, as the latter shoots up and becomes clothed with blanches and leaves, from 

 which water is exhaled during healthy vegetation; the former grows onward, still renew- 

 ing the tender hygronietrical tissue, through which the absorption required to restore that 

 which is lost by elaboration, or consumed by growth, is principally effected; hence the 

 danger of removing trees during the summer season, or when the roots are in rapid action. 

 The growth of the branches and roots being simultaneous, while new branches and leaves 

 are developing — the roots are extending at a corresponding rate, and greatly increasing 

 the absorbing points, they cannot now be removed with safety to the tree, and at the very 

 time when their aid is most required. But when the growth of the season is over, the 

 leaves grow languid, and the rootlets also cease to grow, as the tissue of their extremities 

 not being renewed, gradually becomes hardened, and loses its absorbing powers. This 

 marks the season for transplanting, (namely,) before the growth of tlie season has com- 

 menced, or in the fall, after it is made. This elongation of the roots by their growing 

 points alone, is admirably adapted to the situation in which they are placed, growing, as 

 they do, in such an unequal medium as the soil. If the roots increased in the same man- 

 ner as stems, they would be forced wherever the elongating force was insufllcieut to over- 

 come the obstacle, or wherever this force was most powerful, and they would be thrown 

 into all kinds of contorted shapes, very ill adapted to perform the services for which they 



* See Gray's Botanical Text Book, new edition, pi 31. 

 No. X. 2, 



