CH, in. DOWIsl\^\KD GROWTH OF THE ROOTS. 83 



of Knio'ht and Dutrochet respecting tlie growth of the action 



^ ' of light 



adhesive plants, and the tendrils of clmibins^ plants, from turges- 



<^ i. ' cence, pro- 



the light, we might attribute the upright grow^th of the ^frecithe 



stem and the aberration of its branches from the per- fheVeadof 



pendicular to the mere swelling (turgescence) of the ^ ^ 

 cellular tissue of the new green shoot, and to the action 

 of light on the swelling. 



It is asserted, on microscopic observation, that the CeUuiar 



formation 



bark of all new green shoots is entirely composed of of bark 



^ of greeu 



two layers of cellular tissue. The cells of the outside 

 layer of tissue decrease in size from within outwards ; 

 the cells of the inside layer decrease in size from with- 

 out inwards. Thus the largest cells of each layer are 

 next one another, or in the middle of the bark, and the 

 smallest cehs are on the two sides, that is, the outside 



and the inside of the bark. Owing to this formation, How tur- 

 gescence 



swelling would stretch a slip of bark perfectly straight, ^^^J^/^® 

 since each layer of tissue would tend to curve itself ^0^^^*^°^ 

 outward. But in plants w^hich grow towards the light 

 the inside layer of tissue is the thickest, and therefore 

 the most powerful in its action ; consequently swelling 

 would bend a sHp of such bark inwards or towards its 

 shoot, and such a shoot would be held up by the in- 

 ward pressure of its bark all round it, as a wall may be 

 propped from both sides. But in the bark of plants, 

 or parts of plants, which grow from the light, such as 

 ivy, the tendrils of climbing plants, &c., the outside 

 layer of tissue is the strongest ; and the tops of the 

 shoots of such plants tend to stand upright by the puil- 



Q 2 



