ACTION OF GRAVITATION ON GROWTH IN LENGTH. 845 



grow in a flat curve concave upwards. But external causes may also act in oppo- 

 sition to geotropism even when this is very strongly developed. Thus Knight and 

 Johnson have shown, as I have recently described more in detail, that primary roots 

 with strong positive geotropism, as well as secondary rootlets, when growing in 

 moderately damp air, deviate from their vertical or oblique direction when there is a 

 moist surface near them\ Under these circumstances a curvature concave to the 

 moist surface takes place at the region below the apex where there would otherwise 

 be a downward curvature, the apex being by this means conducted towards the moist 

 surface so that it may penetrate into the moister soil or grow in contact with it. 

 The apparatus represented in Fig. 483 is well adapted to exhibit this phenomenon. 

 It consists of a zinc frame a a covered below with wide-meshed network, thus form- 

 ing a sieve hanging obliquely and filled with moist sawdust//! The seeds ggg 

 germinate in the sawdust, their roots penetrating at first vertically downwards into it. 

 When the apex of a root escapes through the network into air, which is not too 

 dry, it turns towards the moister surface h-m, its geotropism being thus evidently 

 overcome. 



The foregoing account is intended to give the reader a general idea of the various 

 debatable points which are especially to be remembered in the study of Geotropism 

 and to which frequent reference is made in the literature of the subject. Until recently 

 there were no complete observations or measurements of the growth which necessarily 

 accompanies geotropic curvature, or as to the true form of the curvature and its 

 relation to time and other conditions, which might give some clue as to the nature 

 of the internal changes which effect externally the upward or downward curvature. 

 I have endeavoured to supply these in the papers mentioned at the beginning of this 

 section. The observations were made upon organs of which the geotropism was well- 

 marked, such as erect growing stems, the nodes of grass-haulms, and downward-growing 

 tap-roots. 



I. The upavard curvature of stems <which normally gro^ erect'^. My observations were 

 made for the most part on the thick, firm, long internodes of scapes which attain a con- 

 siderable height in a short time, the smooth surface of which can be marked with 

 Indian ink and allows of accurate measurement of the portions thus indicated. The 

 measurements of straight shoots as well as of the convex and concave sides of curved 

 ones were made by means of flexible measures of stout paper upon which the scale 

 was printed. 



In order to be in a position to form an opinion as to the phenomena connected with 

 the upward curvature of stems or internodes placed horizontally, the distribution of 

 growth in these organs must first be understood. A general account of this was given 

 in Sect. 17. At first the whole internode, or a shoot consisting of several internodes, 

 is undergoing elongation. At a later period growth ceases at the base of the shoot, and 

 only a certain number of internodes lying below the terminal bud (this bud is not 

 taken into consideration here) constitute the region of the shoot which is growing and 

 which is capable of making a geotropic curvature. In the case of single internodes, 

 the region in which growth is to continue may lie near either to the base or to the 

 apex ; apical growth is the usual, basal growth the more uncommon case. It is remark- 

 able that similar internodes of closely allied plants behave differently in this respect : 

 thus in the scapes of Allium atropurpureum there is apical growth, whereas in that of 



^ [This exhibition of sensitiveness to moisture has been termed ' Hydrotropism.' (See Darwin, 

 Movements of Plants, p. 180).] 

 * Sachs, Flora, 1873, No. 21. 



