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 up'ward cur'vature of stems 'which normally grow 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 grov/th 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).] 
2 Sachs, Flora, 1873, No. 21. 
