LECTURE XXXII. 



THE COURSE OF GROWTH DURING ELONGATION. 

 PERIODIC VARIATIONS. 



We have hitherto treated of growth only in so far as it is concerned with 

 the production of configuration — of the external and internal form of the organs, and 

 through this of the whole plant. In the present lecture we are to be concerned 

 with growth from an entirely different aspect. We place before ourselves the question, 

 in what manner does the growth of any small portion of a plant run its course when 

 it passes over from the embryonic condition into that of elongation, in order subse- 

 quently to reach the stage where its external growth ceases ? 



Here again, in order at once to keep in view a concrete case, and to avoid the 

 tedious enumeration of the manifold combinations in the distribution of growth in 

 different plants, we may first confine our remarks to the typical shoots and roots which 

 have a terminal growing-point ; and, in order to limit the problem still more, we may 

 first concern ourselves exclusively with the increase in length of such organs — 

 i. e. with growth parallel to the axis of growth '. 



As the most important result, it is first to be insisted upon that every part of a 

 plant, and in fact every transverse disc of it, however short, elongates slowly at first, 

 then, with increasing rapidity, attains a maximum velocity of growth, and then again 

 grows slowly and still more slowly, until the growth finally ceases altogether. In 

 order to illustrate this in as simple a case as possible, we may take the young long 

 primary root, about 2-4 cm. long, of a seedling of Vicia Faha (the common Broad 

 Bean). Two transverse lines of Indian ink are drawn immediately above the 

 growing-point, by means of a small sharp camel-hair pencil, so that they are exactly 

 I mm. apart ; this then denotes the length of a transverse disc of the root. The 

 seedling being now left in a place where it can at the same time absorb water and 

 respire energetically, and therefore where growth takes its normal course at the 

 expense of the assimilated reserve-materials, it is only necessary to measure the 

 length of the transverse disc (originally i mm.) at certain intervals of time, in order to 



' The modem fundamental work on growth in length, is my treatise, ' t^ber den Einjluss der 

 Lufttemperatur und des Tageslichts auf die stiindlichen und tdglichen Anderungen des Ldngen- 

 wachsthums der Intemodien' in Arb. des bot. Inst, in Wzbg., B. I, pp. 99, &c., where also all the 

 then existing literature is thoroughly and critically incorporated. On the growth in length of roots, 

 detailed statements and quotations are found in my treatise ' Uber das Wachsthum der Haupt- und 

 Nebenwurzeln^ op. cit. p. 385 (1873 and 1874). The periodic variations of growth first brought 

 into notice in my first-named treatise were subsequently investigated in detail by Oscar Drude 

 (Leofoldina, B. XXXXVIII, No. 3, Halle, 1881) in the leaves of Victoria regia: he obtained no 

 better information as to the cause of them, however. 



