GROWTH IN THICKNESS. AND GROWTH-CONTRACTION 13 



a protection from the external world. The young stems of grasses and 

 of Cannas are unable to bear the weight of the upper parts when the 

 sheaths are removed, and hence sink downwards. 



In the leaves of Canna and Tulipa, as well as in the stems of grasses, 

 large transpiring surfaces lie above the vegetative zones. Hence large 

 quantities of water and of food-materials must pass through the latter. 

 It is probably of considerable importance that these growing zones, although 

 they consist mainly of embryonic cells, also include a few spiral and 

 annular vessels, and that the total length of the zone is always small. 

 Further research is however necessary to determine the precise manner 

 in which the necessarily rapid transmission of water and food-materials 

 takes place through these meristematic zones *. 



SECTION 4. Growth in Thickness and Growth-contraction. 



The grand period of growth in thickness can be determined from 

 observations on the progress of development 2 . These show that in the 

 trees of temperate regions the annual grand period of growth of the xylem 

 begins, and also ends, sooner than that of the phloem 3 . That the cambium 

 should produce new tissues on both sides, and that it should be pushed 

 outwards by the growth of the xylem cylinder, are facts which call for no 

 special comment. The same happens in fact in many intercalary vegetative 

 zones, and the growing apex of the root produces dermatogen segments on 

 one side, and periblem and plerome segments on the other, while the latter 

 by their subsequent growth push the root-apex onwards. 



A definite correlation exists in each specific case between growth in 

 thickness and growth in length. In some cases both are equally active, 

 whereas in most stems and roots active growth in thickness begins in regions 

 which have ceased to elongate. Occasionally a slight decrease in the 

 diameter takes place after the commencement of active elongation, and 

 similarly growth in thickness may be accompanied by a shortening of the 

 organ affected. This latter occurs to a pronounced extent in many roots, 

 those parts which have ceased to elongate contracting in length as secondary 

 growth commences. 



1 Cf. A. Nathansohn, Jahrb. f. wiss. Bot., 1898, Bd. xxxil, p. 671. 



2 Cf. de Bary, Anat., 1877; Haberlandt, Physiol. Anat, 1896, 2. Aufl., p. 490. Also 

 M. Nordhausen, Beitr. z. wiss. Bot. v. Fiinfstiick, 1898, n, p. 356 ; Schwarz, Physiol. Unters. ii. 

 Dickenwachsthum von Pimis sylvestris, 1898. On the grand period of growth in trees see Th. 

 Haitig, Lehrb. d. Anat. u. Physiol., 1891, p. 264; Jost, Ber. d. Bot. Ges., 1892, p. 587 ; Christison, 

 Bot. Jahresb., 1894, I, p. 223; Reuss, Bot. Centralbl., 1893, Bd. LV, p. 348; Schwarz, I.e. On 

 the grand period of growth in thickness in other plants see Montemartini, 1. c., 1897, p. 15; 

 Macmillan, American Naturalist, 1891, p. 465 (Potato) ; F. Darwin, Ann. of Bot., 1893, Vol. XXVIII, 

 p. 459 (Cucumber). 



3 Strasburger, Bau u. Verrichtung d. Leitbahnen, 1891, pp. 482, 500; Hiilscher, Bot. Centralbl., 

 1883, Bd. xv, p. 303. 



