IV Movement mid Nervous Action in Plants 71 



The tip of the radicle is a very sensitive structure : "it 

 was excited by an attached bead of shellac weighing less 

 than -^^ of a grain ; " it bends towards moisture and 

 away from light ; it always responds to the attraction of 

 earth even when grown in entirely abnormal conditions. 

 " If the tip be lightly pressed or burnt or cut, it transmits 

 an influence to the upper adjoining part, causing it to bend 

 away from the affected side ; and, what is more surprising, 

 the tip can distinguish between a slightly harder and softer 

 object, by which it is simultaneously pressed on opposite 

 sides." " It is hardly an exaggeration," Darwin concluded, 

 " to say that the tip of the radicle thus endowed, and having 

 the power of directing the movements of the adjoining parts, 

 acts like the brain of one of the lower animals." 



But the movements of seedlings are not confined to the 

 radicle. From the seed there also emerges the young stem 

 or plumule, almost always bent in the form of an arch, the 

 tender tip remaining within the protecting seed-coats until 

 the first obstacles to growth have been overcome. If the 

 seed be buried the arch breaks through the ground, and is 

 helped in doing this by slight movements. The young 

 stem grows blindly but unerringly upwards towards light 

 and air, as the root downwards towards moisture and dark- 

 ness ; we cannot yet give any full or sufficient mechanical 

 explanation why, though it is here only too easy to cloak 

 our ignorance under learned nomenclature. ^ 



As the arch grows upwards, freeing itself from the soil, 

 the seed-leaves or cotyledons, if not already in use as store- 



1 It does not seem necessary to encumber this narrative or the 

 student's mind at this stage with the numerous technical terms appHed 

 to movements in relation to gravitation, hght, moisture, etc., since 

 these are only too apt to correspond to no definite mental images of 

 any kind, but be used as mere illusory explanations in terms of "in- 

 herent properties." See next chapter. 



