70 THE DEVELOPMENT OF RAW COTTON 



THE SEED-COATS (Fig. 13). The two coats of the ovule, 

 whose origin we have described, consist of almost un- 

 differentiated cells, all resembling one another, excepting 

 that inner and outer epidermis can be recognized on each 

 coat, and that vascular tissue, or veins, traverses them to 

 provide food and water. The vascular tissue enters the 

 seed by its stalk, runs along the side, and then breaks up at 

 the wide butt of the seed into short distributing branches. 



The two coats, though originating separately, are 

 closely appressed to one another, and for all practical 

 purposes form a single layer . In the ripe seed this double 

 jacket of delicate thin-walled cells has been converted 

 into a horny protective envelope, consisting of the fol- 

 lowing structures from the outside inwards: There is an 

 epidermis (between the cells of which the lint arises), an 

 outer pigment layer, a hard "crystal layer" (Krystall- 

 schicht of Weisner), then a very thick horny layer of 

 palisade-like cells, followed internally by the inner pig- 

 ment layer, to which succeed the two papery remnants 

 already mentioned as being derived from the endosperm, 

 etc. 



Though their thickness becomes greater through in- 

 creased cell size and division, the coats show no marked 

 change until the twelfth day of boll development, except- 

 ing for the growth of the lint and fuzz hairs. 



On the twelfth day the outer epidermis of the iimer 

 seed-coat (corresponding to the layer from which the lint 

 arises in the outer seed-coat) begins to increase the size 

 of its constituent cells. 



By the fifteenth day these cells have undergone marked 



