GROWTH IN LENGTH UNDER CONSTANT EXTERNAL CONDITIONS. 735 



than that given by Knight himself, who thought the movement of the sap in the wood 

 was promoted by the swaying of the stem caused by the wind. 



The great assistance to the increase in diameter of trees afforded by the diminution 

 of the pressure of the bark on the cambium was long ago employed in horticulture. 

 The bark of young trees is split from above downwards in summer; cushions of 

 wood are formed at the edges of the incisions, which soon close up the wounds. 

 The use of this process is that from the more rapid increase of the wood in thickness, 

 the conduction of water to the leaves becomes more copious and the loss by transpir- 

 ation is more easily replaced. The development of the buds and hence the formation 

 of the organs of assimilation will be promoted by the increase of turgidity in the young 

 branches. 



Sect. 17. Course of the growth in length under constant external con- 

 ditions \ It has already been explained in the morphological portion of this work 

 that the organs of a plant do not grow simultaneously and uniformly at all points ; 

 but that roots and stems always increase slowly in size at the apex, as leaves also do 

 at least at first. The growing cells not only multiply by cell-divisions which take 

 place regularly, but do not as a whole exceed a certain size, which is always small. 

 Below this punciiim vegeiationis, consisting of primary meristem, not only does the 

 differentiation of the homogeneous tissue into layers of different kinds begin, but 

 also a more rapid increase in size of the cells, which do not now divide so often as 

 before. In the parts of the organ which lie further from the pjinctiu7i vegelationis 

 cell-division ceases altogether (but at different periods in the different layers of 

 tissue), while the growth of the cells still actively continues, until at length, when 

 they have attained their ultimate form and size, the growth of the whole ceases. 

 The cells are then several hundred or even thousand times larger than at the time 

 of their formation beneath the punctum vegetatioiiis. When the growth of stems, 

 leaves, and roots has reached a sufhciendy advanced stage of development, we are 

 able therefore to divide their tissue into three regions : — (i) the piinchim vegeiationis, 

 where new cells are chiefly formed and increase in size is slow; (2) the portion 

 where the main part of the increase in size takes place, but where there is no longer 

 any cell-division or only to a subordinate extent ; this is the elongating portion of 

 the organ; and (3) the portions which no longer grow, at least in length, i.e. the 

 mature portions of the organ. When growth entirely ceases at the punctum vege- 

 tatiojiis, as is usually the case with leaves, all the cells continue to enlarge until the 

 whole is mature. If the stem produces a number of closely crowded leaves, as it 

 usually does at its growing end, the whole of the region in which the chief part 

 of the cell-division takes place is clothed with young leaves, which also themselves 

 consist of cells undergoing division. But as soon as the leaves enter the second 

 stage of development and begin to lengthen, they incline outwards; and when 

 the stem is growing rapidly in length and forming evident internodes (which is 

 by no means always the case) the lengthening begins at those points where it bears 



^ Ohlert, Langenwachsthum der Wurzel, Linn?ea 1837, vol XI, p 615. — Miinter, Bot. Zeitg. 

 1843, p. 125, and Linneea 1841, vol. XV, p. 209. — Griesebach in Wiegmann's Archiv. 1843, p. 267. — 

 Sachs, Jahrb. fiir wissensch. Bot. i860, vol. IT. p. 339. — Miiller, Bot. Zeitg. 1869, No. 24. — Sachs, 

 Arbeit, des Bot. Inst, in Wurzburg 1872, Heft II p. 102; ditto, Heft III, 1873, and Flora 1873, 

 No. 21. — Askenasy, Flora 1873, No. 15. 



