744 A TEXTBOOK OF THEORETICAL BOTANY 



Embryogeny 



Development begins with a period of free nuclear division, followed by 

 wall formation, and the whole contents of the archegonium become cellular. 

 This mass of undifferentiated cells is known as the protocorm and has a 

 parallel in some Cryptogamic types such as Lycopodinm. There is no special 

 suspensor development but the cells in the neck region enlarge while those 

 near the base become numerous and remain small. It is from these latter 

 that the embryo develops. It is quite straight and dicotyledonous (Fig. 747), 

 but there is a precocious formation of foliage leaves from the plumule and 

 at germination there are usually five present. 



The embryo lies embedded in the prothallial tissue which serves as 

 endosperm. It is reported that the micropylar end of the protocorm grows 

 out into the endosperm and may act as an haustorium, but this is uncertain. 



In the interval between May and September the ovules often fall from the 

 tree. Fertilization and embryo development then proceed while they are 

 lying on the ground. This may perhaps account for the strange absence of 

 embryos in the fossil seeds of Pteridosperms. If embryo formation were very 

 much delayed and then proceeded continuously, with no resting phase, it is 

 scarcely to be expected that we should find embryos in the seeds. 



As the embryo matures the integument also enlarges and the outer flesh 

 thickens, producing a mature " fruit " about an inch long (Fig. 748). The 

 inner flesh becomes papery but the outer flesh is very mucilaginous and 

 has an unpleasant smell. The " kernel," i.e., the endosperm, is, however, 

 edible. 



The symmetry of the seed is slightly bilateral, corresponding to the oval 

 section of the prothallus, and the longer diameter is marked by two prominent 

 ridges on the hard testa. Seeds with three ridges sometimes occur, which 

 strongly resemble the Medullosean fossil seed Trigonocarpus. 



Usually only one of each pair of ovules develops to maturity, though 

 exceptions are not infrequent. 



Germination 



The two cotyledons are somewhat unequal : the larger is slightly notched 

 at the apex but the smaller is cleft nearly into two. In each lobe there are 

 single bundles which unite at the base of the cotyledon into a double bundle 

 with mesarch structure. This is almost the only relic in Ginkgo of the 

 mesarchy which marks the foliar bundles of Cycas. Its presence here supports 

 the belief that seedling structure may retain ancient characters and shows 

 that Ginkgo has departed further from the primitive Fern stock than have 

 the Cycads, at least in some respects. 



The cotyledons are hypogeal and remain persistently embedded in the 

 nutritive endosperm. There is a primary tap root, and the young leaves 

 are deeply bilobed (Fig. 749). A point of interest is that the young leaves 

 during development are bent over at the apex and the margins are rolled in 



