i54 SENSITIVENESS TO LIGHT. CHAP. IX 



it is no doubt a great advantage to them in then 

 struggle for life to expose their cotyledons to the 

 light as quickly and as fully as possible, for the sake 

 of obtaining carbon. It has been shown in the first 

 chapter that the greater number of seedlings circum - 

 nutate largely and rapidly ; and as heliotropism con- 

 sists of modified circumnutation, we are tempted to 

 look at the high development of these two powers in 

 seedlings as intimately connected. Whether there are 

 any plants which circumnutate slowly and to a small 

 extent, and' yet are highly heliotropic, we do not 

 know ; but there are several, and there is nothing 

 surprising in this fact, which circumnutate largely and 

 are not at all, or only slightly, heliotropic. Of such 

 cases Drosera rotundifolia offers an excellent instance, 

 The stolons of the strawberry circumnutate almost 

 like the stems of climbing plants, and they are not at 

 all affected by a moderate light; but when exposed 

 late in the summer to a somewhat brighter light they 

 were slightly heliotropic; in sunlight, according to 

 De Vries, they are apheliotropic. Climbing plants 

 circumnutate much more widely than any other plants, 

 yet they are not at all heliotropic. 



Although the stems of most seedling plants are 

 strongly heliotropic, some few are but slightly helio- 

 tropic, without our being able to assign any reason. 

 This is the case with the hypocotyl of Cassia tora, and 

 we were struck with the same fact with some other 

 seedlings, for instance, those of Reseda odorata. With 

 respect to the degree of sensitiveness of the more 

 sensitive kinds, it was shown in the last chapter that 

 seedlings of several species, placed before a north-east 

 window protected by several blinds, and exposed in 

 the rear to the diffused light of the room, moved 

 with unerring certainty towards the window, although 



