GINKGOALES 



20I 



being found in only one ovule out of fifty at this stage. The deriva- 

 tion of the mother cell from a hypodermal archesporial cell could not 

 be traced. It is recognizable in the midst of the so-called "spongy 

 tissue," which may or may not be sporogenous in origin (figs. 229, 

 230). Pax (13) evidently regarded this tissue as representing a 

 many-celled archesporium; but whatever its morphological nature, 

 its tapetal function is clear. In the mature mother cell there appears 

 the peculiar "kinoplasmic mass" 

 observed in the mother cells of 

 Taxus, Larix, Taxodium, Thuja, 

 and certain other genera. The 

 reduced number of chromosomes 

 is eight; and although a linear 

 tetrad is usual, one case of bilateral 

 arrangement was observed, and 

 two cases in which the upper cell 

 of a row of three had divided longi- 

 tudinally. 



The history of the tapetal zone 

 is as follows. During tetrad-for- 

 mation, this glandular-looking tissue 

 increases in bulk by cell division, 

 encroaching actively upon the sur- 

 rounding nucellar tissue until the 

 last week of June. The absorbing 

 cells soon become vacuolate and 

 finally multinucleate (fig. 231). The megaspore membrane surround- 

 ing the gametophyte now shows the characteristics of its mature con- 

 dition, although it has not reached its maximum thickness. After 

 June, the tapetal zone in turn begins to be destroyed by the encroach- 

 ment of the young gametophyte (endosperm), which at this time is 

 still in the free nuclear stage, but the distinct membrane about it 

 has been formed. By the time the centripetal growth of the gameto- 

 phyte is about one-third of the distance across the embryo sac, all 

 of the tapetal zone has been crushed. It is evident that this zone 

 of cells is one that digests and absorbs, performing the "tapetal 

 function" for the mother cell and the early stages of the gametophyte. 



Fig. 231. — Ginkgo biloba: multi- 

 nucleate stage of spongy tissue; bor- 

 dering upon the spongy tissue at the 

 left is the conspicuous megaspore 

 membrane, and just beyond are three 

 of the free nuclei of the endosperm; 

 X650. — After Miss Carothees (46). 



