MEDULLOSEAE 453 



consists of two clearly distinct layers — the outer layer 

 or sarcotesta, composed of delicate, partly lacunar tissue, 

 bounded externally by a sharply differentiated hypo- 

 derma and epidermis, and the inner ribbed sclerotesta, 

 constructed, like the stone of a peach, of dense, thick- 

 walled tissue. The ribs show a very definite arrangement. 

 There are three principal ridges, corresponding to 

 sutures, in the sclerotesta, and usually three secondary 

 ridges in each space between the former, making twelve 

 ribs in all (Fig. 171); within the sclerotesta there are 

 some traces of an inner soft layer. The nucellus has a 

 definite epidermis, and appears to have been free from 

 the integument, from the chalaza upwards; it terminates 

 at the apex in a dome-shaped pollen-chamber, provided 

 with a long, narrow beak (Fig. 1 70, p.c, p.c.b), as in the 

 seeds of the Cordaiteae, described in Chap. XII. The 

 membrane of the megaspore or embryo-sac is evident, 

 but the prothallus has not yet been found preserved. 



The vascular system of the seed is double. At the 

 base six bundles branch off from the common supply- 

 strand and pass upwards through the sarcotesta (Fig. 

 1 70), taking a definite position opposite certain of the 

 secondary ridges (Fig. 171). These bundles appear to 

 have ,beerf collateral, with external phloem, and there is 

 evidence that the xylem was mesarch. The inner 

 vascular zone formed a complex tracheal network in 

 the nucellus ; near the chalaza the sheath of nucellar 

 tracheides is continuous ; farther up they range them- 

 selves in longitudinal strands connected by abundant 

 transverse anastomoses. The mos^ remarkable feature 

 of the seed is the long micropylar tube, formed by an 

 extension of the ribbed sclerotesta, and enclosed in a 



