THE EMBRYO AND SEEDLING 



131 



destined to form nothing but protective scales. When 

 the seedKng has completed its intraseminal development, 

 as the development before it breaks through the seed 

 coats is called, it usually has one foliage leaf well started 

 and the rudiments of one or two more are easily dis- 

 tinguishable. 



The root structures of the 

 cycad embryo are rather com- 

 plex. At the stage shown in 

 Fig. 69 there is a conspicuously 

 swollen portion just back of 

 the cotyledons. This portion is 

 called the "coleorhiza," because 

 it acts as a protecting shield for 

 the tender root until the stony 

 layer of the seed coat has been 

 ruptured. 



Some features of the mature 

 seed are common to all cycads 

 (Fig. 70). There is an outer fleshy layer, which may 

 persist for months after the seed begins to germi- 

 nate; a middle stony layer, which is as hard as that of 

 a hickory nut but at the same time slightly elastic ; and 

 an inner fleshy layer, which is thin, dry, and mem- 

 branous. The endosperm is firm but cuts easily with a 

 knife. Its cells are richly stored with starch but contain 

 other materials, some of them doubtless the poisonous 

 elements which have made so much trouble. 



Fig. 70. — Dioon edule: 

 section of mature seed, 

 showing tlie embryo with 

 two cotyledons, the endo- 

 sperm (dotted), the stony 

 layer of the seed coat 

 (shaded), and the outer 

 fleshy layer. Natural size. 



THE SEEDLING 



The term seedling is applied for an indefinite period 

 beginning with the rupture of the seed coats. As a 



