194 THE LIFE OF THE PLANT 



results have been subsequently obtained, so that the 

 question as to the mode of action of the force of gravity 

 upon the root must be considered as still open.^ Further 

 on we shall see that this question becomes considerably 

 complicated, and that in order to explain the phenomenon 

 we must take into account not only the structure of 

 entire organs, or of the tissues that form them, but also 

 the details of the structure of the cells forming these 

 tissues. 



Now let us see what other external conditions influ- 

 ence the phenomena of growth. In looking for the 

 causes which determine the natural, vertical direction 

 of growth of both the stem and root, we have proved 

 that it does not depend on the light. Further, we have 

 seen that growth is possible even when light is com- 

 pletely absent : potatoes and turnips put out long shoots 

 in the total darkness of cellars. The same can be 

 proved by experimenting with any seed or shoot ; they 

 all will grow in the dark. 



Are we entitled to infer from this that light does not 

 influence growth ? Not in the least. A very simple 

 experiment will prove how considerable this influence is. 

 If we let cress seeds germinate in two pots filled with 

 exactly similar soil, placing one of them in the dark 

 and leaving the other in the light, the difference will 

 not be long in showing itself. Cress grown in the dark 

 will sometimes be ten times longer than that grown in 

 the light ; but its stems will be slender and unhealthy, and 



1 For instance, the considerations generally brought forward by 

 botanists concerning the growth of the root-tip in mercury are far from 

 being convincing, two different phenomena having been confounded in 

 these experiments : growth (of the whole organ in length) and curvature 

 (depending only upon the difference in growth of the upper and lower 

 sides of the curving part). A root exerts its pressure first of all because 

 it grows, and it is quite obvious that this pressure caused by growth has 

 nothing to do with the weight of the root, any more than the weight of 

 the flexible rod (fig. 54, II d) when overcoming a certain resistance and 

 at the same time bending passively downwards. 



