CYCADALES 



109 



The upper part of the ovule, at the time of fertiHzation, is shown 

 in detail in a later figure (fig. 147). This figure shows the nucellus 

 with pollen tubes which have digested their way completely through. 

 The sharp beak is characteristic. At this stage the inner fleshy layer 

 has become a thin, dry membrane, sticking to the inner border of the 

 stony layer. There are many mucilage ducts in the outer fleshy layer 

 and many cells (shaded in the drawing) filled with tannin. 



The younger stages in the development 

 of the ovule have not been studied very 

 thoroughly on account of the difficulty in 

 getting material. In most cycads these 

 stages occur while the young cone is 

 still covered by scale leaves and there is 

 uncertainty whether a cone or a crown 

 of leaves is developing. Naturally, in 

 conservatories, it would be difficult or 

 impossible to get permission to cut out 

 the top of a rare plant. There are few, if 

 any, trained histologists in cycad regions, 

 and when a histologist reaches such a 

 place, on a hasty trip, the young stages 

 might not be available. The most thor- 

 ough study was made by Dr. F. Grace 

 Smith, s^s who, after sending repeatedly 

 and getting little except to learn the ap- 

 proximate time for various stages, went 

 and spent a month in the Zamia floridana 

 region, and fixed material. Previously, 

 Lang^-*^ had figured a row of three cells in 

 Stangeria, the lowest of which was cer- 

 tainly a megaspore, and Treub^''^ j^a^fj found a similar stage in Cera- 

 tozamia. Dr. Smith found, quite regularly, a row of four megaspores 

 in Zamia, three of which aborted, while the fourth germinated and 

 formed the functioning female gametophyte (figs. 103, 104, 105). 



The formation of the megaspore brings to a close the 2X genera- 

 tion, which in all plants, from the Bryophytes up, and also in many 

 Thallophytes, is also the sporophyte generation. The reduction of 



Fig. ioi. — Dioon edule: lon- 

 gitudinal section of ovule, 

 shortly after pollination; m, 

 micropyll; n, nucellus; e, endo- 

 sperm; s, stony layer; p, basal 

 papilla; i, inner vascular bun- 

 dles; 0, outer vascular bundles; 

 a, abscission layer; X 2. — After 

 Chamberlain.'"* 



