128 



MORPHOLOGY OF GYMNOSPERMS 



istic beak, which is forced up into the micropyle; while in the center 

 of the beak and below it cells break down and form the beginning 

 of the pollen chamber. After the reception of the pollen the beak 

 closes and hardens, effectively shutting in the pollen grains; while 

 the pollen chamber continues to enlarge for several months by the 



breaking-down of adjacent tissue, until 

 it finally extends entirely through the 

 nucellus, leaving no obstruction be- 

 tween the pollen tubes and the female 

 gametophyte (figs. 121, 122). 



The integument consists of three 

 easily recognized layers : an outer and 

 an inner fleshy layer of rather simple 

 structure, and a very complex stony 

 layer between them. Many of the 

 cells of the stony layer are elongated 

 in various directions and are often 

 curved, thus making not only a very 

 hard but also a very tenacious tissue. 

 The two sets of vascular bundles are 

 found in the fleshy layers. Whether 

 the single integument has always been 

 single or has arisen from the fusion-of 

 two structures, as if the cupule of a 

 Lagenostoma had fused with the in- 

 tegument, is an open question. The 

 cupule of Lagenostoma was doubtless 

 fleshy, and contained vascular bundles 

 which have been compared with the 

 outer bundles of the cycad ovule; 

 while the single fleshy layer, within the stony layer, contained the 

 inner bundles. Structurally, however, the stony layer is different, 

 being a modified epidermis in Lagenostoma, and a complex tissue in 

 the cycads. It is doubtful whether there is any homology between 

 the cupule of Lagenostoma and the outer fleshy layer of the cycads. 

 If there is a boundary between the inner fleshy layer and the nu- 

 cellus, it is not conspicuous, but the innermost layer of cells next the 



Figs. 121, 122. — Dioon edule: 

 general view of the nucellus with 

 pollen tubes; fig. 121, pollen cham- 

 ber and pollen tubes (November 20) 

 about two months after pollination; 

 X40; fig. 122, pollen tubes (in 

 May) after the pollen chamber has 

 extended entirely through the nu- 

 cellus; Xio. — After Chamberlain 

 (63). 



