634 THE STRUCTURE OF ECONOMIC PLANTS 



thick-walled layer. The suberized semi-permeable membrane 

 referred to above, which is a part of the wall of the inner epidermis, 

 lies adjacent to the endosperm. In most parts of the seed, the 

 endosperm is two cell layers in width; but, adjacent to the tip 

 of the primary root, it may be three or four cells in thickness. Its 

 cells are thick-walled; and, in some cases, projections from the 

 walls extend into the cell cavities which are filled with fatty 

 substances and protein granules. 



Fig. 331. A, surface and side views of akene ; B-E, stages in development of seedling ; F, 

 outline of cotyledon showing pattern of main veins ; G, transection through akene showing 

 two cotyledons surrounded by endosperm, seed, and fruit coats, variety New York Regular. 



Development of the Seedling. — The optimum germination of 

 lettuce seed is obtained where there is an adequate supply of 

 water, a temperature not in excess of i5° C, and good aeration. 

 Borthwick and Robbins secured the best results when the seed 

 was stored in moist folds of burlap at 4° C. for a period of four to 

 six days preceding planting. The percentage of germination falls 

 rapidly at temperatures above 15° C. and may be completely 

 inhibited at 30° C. in most varieties. The experimental evidence 

 indicates that this inhibition is due to the structures surrounding 

 the embryo, which consist of the two-layered endosperm and the 

 semi-permeable membrane of the integument. These are thought 



