INFLUENCE OF GRAVITATION ON GROWTH. 759 



primary stems of seedlings ; much less strongly in the secondary roots which spring 

 from rhizomes, climbing stems, &c. The secondary roots of the first and of higher 

 orders which spring from the primary roots of seedlings display this phenomenon 

 in different degrees. It appears to be the general rule that when lateral shoots of 

 the same kind spring from a vertical and therefore decidedly geotropic organ, the 

 branches of the first order are less geotropic, and the further ramifications still less 

 so the higher the order to which they belong ; the exceptions to this rule may be 

 caused by special circumstances. This gradation is very obvious in roots. From the 

 primary root or a strong root springing from the stem with decidedly positive 

 geotropism, proceed secondary roots of the first order which exhibit the phenomenon 

 much less decidedly; and from these again secondary roots of the second order 

 which apparently are not at all geotropic, and therefore grow in all directions as they 

 may chance to originate. Geotropism, like hehotropism, does not depend on the 

 organ containing or not containing chlorophyll, nor on whether it consists of masses 

 of tissue or of a simple row of cells or of a single cell. To this last category belong, 

 for example, the positively geotropic radical tubes of the Mucorini and the negatively 

 geotropic sporangiophores of the same family and of numerous other Mould-fungi. 

 In the same manner the rhizoids of Chara display positive, the stems negative geo- 

 tropism, both consisting of unicellular segments, the former destitute of chlorophyll, 

 the latter green. Whether and how strongly an organ is positively or negatively 

 heliotropic or geotropic depends altogether on its importance in the economy of the 

 plant, and hence on its physiological functions. 



From the remarkable fact that there are organs endowed with positive and 

 negative hehotropism and geotropism, and from many similarities exhibited by the 

 two phenomena, the question presents itself whether all positively heliotropic organs 

 must not possess one description of geotropism either positive or negative, or vice 

 versa ; in other words, whether the two properties do not stand in some definite 

 relation to one another. This does not however appear to be the case. Of primary 

 roots, all of which are positively geotropic, some display positive, others negative 

 hehotropism ; and again, the aerial roots of Chlorophytum, Aroidese, and Orchideae, 

 display very distinct negative hehotropism, but are scarcely at all geotropic. There 

 appears therefore to be no necessary connection between the two phenomena. 



It is clear that organs which are both heliotropic and geotropic, and on which, 

 since they lie obliquely to the horizon, the light falls from above or from below, are 

 subject to changes in their growth dependent both on light and on gravitation. 

 Thus, for example, the bending upwards of a branch placed horizontally on which 

 the light falls from above may be caused at the same time by positive hehotropism 

 and by negative geotropism. An erect stem, on the other hand, which turns helio- 

 tropically towards a source of light at the side and thus makes a curvature which is 

 concave below, will have a tendency to become erect in consequence of its negative 

 geotropism, just as if there were no light falling on it from one side. Stems there- 

 fore which in the evening were bent by positive hehotropism, will stand upright in 

 the morning. These considerations are evidently of the first importance in making 

 observations on the two phenomena. 



We have already seen that no clear idea has yet been obtained of the mode in 

 which light acts in influencing growth in heliotropic organs. As little are we at 



