202 



MORPHOLOGY OF GYMNOSPERMS 



The single thick integument, forming a long and narrow micro- 

 pylar tube, is at first homogeneous, but by the last of May three 

 distinct layers have become differentiated. The outermost layer is 

 a tissue of large, thin-walled cells with numerous mucilage cavities, 

 and finally becomes the fleshy layer of the mature testa. The middle 

 layer consists of small isodiametric cells, and later forms the stony 

 layer of the testa. The innermost layer consists of large, loosely 



232 233 



Figs. 232, 233. — Ginkgo biloba: diagrams of ovules showing the stony layer (s) 

 between the outer (0) and inner (i) fleshy layers, and the collar (c); fig. 232, after the 

 stony layer has become hard, but before the inner fleshy layer has become dry; the 

 inner fleshy layer and the nucellus are shaded with lines; fig. 233, after the inner fleshy 

 layer and nucellus have become dry and papery; X2. 



arranged, very thin-walled cells, which in the free portion of the 

 integument are differentiated into two regions : the inner consisting of 

 vertically elongated cells, a tissue restricted to the free portion of the 

 integument; the outer consisting of transversely elongated cells, a 

 tissue that extends the whole length of the ovule just within the stony 

 layer. This inner tissue is at first delicate and watery, but soon 

 becomes crushed and dry, forming in the mature testa a papery layer 

 lining the stone and adherent to it (figs. 232, 233). This general 

 structure of the testa is practically cycadean (p. 128). It has come to 



