MEDICAGO SATIVA 319 



in surface view, while at tlie base the cell is thinner walled and the 

 broad lumen is circular or oval in outline. 



Beneath the palisade cells is a subepidermal layer of cells that 

 are usually broad at the base with intercellular spaces at their outer 

 limits. They are about 6 fx high in most parts of the seed coat and 

 very broad, ranging up to 30 n. These hour-glass or I-shaped cells 

 are characteristic of many legumes and are sometimes referred to as 

 "osteosclerids." Their basal portion is somewhat broader than 

 the outer part, and the thick walls have deep pits and longitudinal 

 striations so that the surface view presents a radiating pattern. 



Under the subepidermal cells is a parenchymatous zone which 

 consists of several layers of much compressed cells that are peri- 

 clinally elongated and thin-walled. The outermost layers are 

 without intercellular spaces while the inner ones are commonly 

 spongy in their arrangement. The endosperm has a distinct outer 

 layer of aleurone cells, and underlying it is a zone of large thin- 

 walled parenchymatous cells which become mucilaginous follow- 

 ing the intake of water. The epidermal cells of the cotyledon are 

 small, and the mesophyll usually consists of three layers of palisade 

 cells and several rows of spongy cells. The cotyledons are high in 

 protein content, and fat and starch may also be present. 



The occurrence of impermeable or "hard" seeds is not uncom- 

 mon. The percentage of such seed in commercial lots may be 

 high enough to constitute a real problem in alfalfa production, as 

 in Colorado, where, over a period of years, it was found that ap- 

 proximately IX per cent of the seed was of that type. The 

 impermeable seeds fail to imbibe water and germinate within a 

 reasonable length of time, and various methods have been used to 

 overcome this condition. These include treatments with hot 

 water, sulphuric acid, scarification, and the application of dry heat. 

 In relating this problem to the structure of the seed coat. Lute (18) 

 determined that it is not the cuticle which is the impermeable 

 layer, but that this characteristic resides in some peculiarity of the 

 outer portion of the palisade cells, although no structural or 

 mechanical differences between permeable and impermeable seeds 

 were discovered. 



Development of the Seedling. — In germination, the primary 

 root emerges near the hilum and penetrates the soil rapidly, forming 

 a slender unbranched tap root. The seedling is epigeal, in contrast 

 to the hypogeal pea; and the oval cotyledons are raised above the 



