GEOTROPISM 121 



(Loeb 542 ). This can be demonstrated if we mark the 

 cortex in definite intervals with india ink at the begin- 

 ning of the experiment (Fig. 35). After some time the 

 distance between these marks will increase in a certain 

 region of the under side, while it remains constant on the 

 upper side, and this difference causes the bending. This 

 positive increase in length of the under side can only 

 happen through growth, and this growth of the cortex 

 on the lower side of the stem takes place at the expense 

 of material furnished constantly by the leaves which send 

 it in the direction toward the basal end of the stem. When 

 we compare the rate of geotropic bending of horizontal 

 stems without leaves and with one or two leaves at the 

 apex, we find that the bending in the latter is much more 

 rapid (Fig. 36), owing to the greater mass of material 

 supplied for the growth of the cortex, and the same is true, 

 if we compare the rate of curvature of stems having a 

 whole apical leaf attached with that of stems having an 

 apical leaf whose mass has been reduced by cutting off 

 parts of the leaf (Figs. 37 and 38). The writer has shown 

 in other experiments that under equal conditions leaves 

 produce material fit for growth in proportion to their 

 mass. It is, therefore, a safe inference that the influence 

 of the mass of an apical leaf upon the rate of geotropic 

 bending is due to the mass of material it sends into the 

 stem. This material has obviously a tendency to behave 

 like a liquid — which it probably is — and to sink to the 

 lower level. It is, therefore, useless to look for a " gravi- 

 tational stimulus." 526 ' 544 



What has been demonstrated in this case explains 

 probably also why the apex of many plants when put into a 

 horizontal position grows upward, and why certain roots 

 under similar conditions grow downward. It disposes 



