GROWTH UNDER CONSTANT EXTERNAL CONDITIONS. 815 



number of the vessels increased, but also that of the cells, so that the more vigorous 

 grovi'th of the cells had induced more numerous divisions. The dry-weight also in- 

 creased with the moisture of the soil : but if the latter exceeded 60 per cent, of the 

 quantity of water necessary for complete saturation, the dry-weight began to diminish. 

 It appears that in this particular also, as in the case of heat and light, there is an 

 optimum, below which every increase is favourable but above which it is injurious. 



Sect. 17. Course of the Growth in Length under Constant External 

 Conditions \ 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 multiply by cell-divisions which take place 

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

 Below this punctum vegetatioms, 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 punctum vegetaiiojiis 

 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 vegetatioms. When the growth of stems, 

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

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

 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- 

 tatioms, 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 

 the leaves, which also begin to lengthen at the same time ; the older mature leaves 

 are generally found on mature internodes. If the internodes are clearly marked 

 off from one another, as is especially the case when the leaves are verticillate or 

 sheathing at their base, each internode forms a more or less individualised whole 



^ Ohlert, Langenwachsthum der Wurzel, Linnsea 1837, ^ol. XL p. 615. — Miinter, Bot. Zeitg. 

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

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

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

 No. 21.— Askenasy, Flora 1873, No 15, [and Verhandl. d. nat.-med. Vereins zu Heidelberg, N. F., 

 Bd. II, 1878, Ueb. eine neue Methode um die Vertheilung der Wachsthumsintensitat zu bestinimen; 

 Strehl, Unters. ueb. das Langenwachsthum der Wurzel, Diss. Inaug., Leipzig 1874.] 



