68 



THE COTTON PLANT IN EGYPT chap. 



the plant, its immediate cause being the formation of a 

 special tissue across the base of the stalk of the leaf, bud, 

 or flower. The facultative position of this tissue, or 

 " absciss-layer," is marked by a slight groove on the stalk. 

 When the determining stimulus has been received by the 

 cells of this layer, which are otherwise indistinguishable 

 from their neighbours, they proceed to divide, and the 

 daughter-cells separate from one another, thus destroying 



plants, 

 weekly. 



Fig. 46. — Composition of Sheddings. 



the continuity of the stalk except in such tissues as the 

 wood. The phloem being among the severed tissues, 

 synthesised food supplies are cut off, the organ dies, sooner 

 or later the stalk breaks, and the organ is shed. Even 

 before any sign of unhealthiness becomes visible, the stalk 

 may break at a light touch, being retained merely by the 

 wood-vessels, bast fibres, and cuticle. 



The reaction to the determining stimulus is very rapid 

 in cotton, on account of the extreme simplicity of the 

 absciss-layer. A convenient way of provoking this 

 unknown stimulus is to cut off a few roots. Within four 

 days after such treatment, we find that complete severance 

 of the tissues of the absciss-layer has taken place. Micro- 



