GROWTH UNDER CONSTANT EXTERNAL CONDITIONS. 815 
number of tlie vessels increased, but also that of the cells, so that the more vigorous 
growth 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 w^hich take place 
regularly, but do not as a whole exceed a certain size, which is always small. 
Below this punctum vegetationis, 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 vegetationis 
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 grow^th 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 vegetationis. 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 vegetationis, 
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- 
tationis, 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 individuahsed whole 
^ Ohlert, Längenwachsthum der Wurzel, Linnsea 1837, '^ol. XI. p. 615. — Miinter, Bot. Zeitg. 
1843, p. 125, and Linnaea, 1841, vol. XV. p. 209. — Griesebach in Wiegmann's Archiv, 1843, p. 267. — 
Sachs, Jahrb. für wissensch. Bot. i860, vol. II. p. 339. — Müller, Bot. Zeitg. 1869, No. 24. — Sachs, 
Arbeit, des Bot. Inst, in Würzburg, 1872, Heft IL p. 102 ; ditto, Heft III, 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 Wachsthumsintensität zu bestimmen; 
Strehl, Unters, ueb. das Längenwachsthum der Wurzel, Di<s. Inaug., Leipzig 1874.] 
