172 EMBRYOGENESIS IN PLANTS 



angiosperm, there is a progressive reduction in the development of the 

 female gametophyte, a loss of motiUty in the male gamete, and 

 elaboration in the retention, nourishment and protection of the embryo 

 on the sporophyte plant and in the maturing of resistant, encapsulated 

 germs. 



The main homologies relating to the embryonic development may 

 be briefly indicated at this point. The megasporangium of Marsilea or 

 SeJagiuella corresponds to the nucellus of Pinus, the protective integu- 

 ment in Pinus being a new development not found in pteridophytes. 

 The nucellus and its integument constitute the ovule which is borne on 

 the upper side of a fleshy scale. In the nucellus of Pinus, as in the 

 pteridophyte megasporangium, meiosis takes place : the four cells with 

 haploid nuclei form a linear series, of which only the innermost 

 develops. It corresponds to the single megaspore in a pteridophyte and 

 becomes transformed into the embryo sac. But whereas the thick- 

 walled pteridophyte megaspore is usually shed at maturity, or soon 

 after, by a rupture of the sporangial wall, the megaspore in Pinus is 

 retained on the parent plant. As Bower (1948) has said, all these facts 

 'point to the conclusion that the thin-walled embryo-sac of Pinus is a 

 retained megaspore, and that we see in it a derivative state which has 

 been universally adopted by seed-plants.' The retention of the mega- 

 spore in the ovule gives it the biological advantage of continued 

 nutrition. Just as the pteridophyte megaspore, e.g. that of Se/aginella 

 or Marsilea produces a prothallus, or female gametophyte, with 

 archegonia at the apical end (or proximal face), so also in Pinus the 

 megaspore or embryo-sac enlarges, its nucleus undergoes many 

 divisions and a cellular prothallus (the so-called endosperm) is formed. 

 Three to six archegonia become differentiated in this prothallus, the 

 necks of the archegonia being directed towards the base of the scale, 

 i.e. towards the axis of the cone. In a majority of living gymnosperms, 

 fertilisation of the ovum within the archegonium takes place by a non- 

 motile male gamete, or male nucleus, which is conveyed to the neck of 

 the archegonium by means of a penetrating pollen-tube. In the 

 Cycadales and Ginkgoales, however, the germinating pollen grains, i.e. 

 microspores, develop tubes, but the male gametes still have the 

 organisation of large motile spermatozoids. These are two in number 

 in Ginkgo, Zamia and Cycas, but they are numerous in Micwcycas. 

 The young embryo grows downwards through the base of the arche- 

 gonium into the prothallus (or 'endosperm') so that the embryo in the 

 mature seed is embedded in nutritive tissue. Thus, in Pinus, as com- 

 pared with heterosporous pteridophytes, the only new structure in the 

 reproductive mechanism is the integument in the ovule, all the other 

 features, as Bower has said, being 'modifications in accordance with 



