412 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



some other genera there are two prothallial cells; and in the Taxa- 

 ceae, Cephalotaxaceae, and most genera of the Taxodiacea and 

 Cupressaceae the prothallial tissue is completely eliminated. 

 Another feature of considerable interest is the great disparity in the 

 size of the two sperm cells in Taxus, Torreya, and Cephalotaxus, 

 the smaller cell presumably being on the way to elimination. Some 

 species of Cupressus are exceptional in having multiple male cells, 

 all of which seem to be capable of functioning. 



Coming to the Gnetales, Ephedra with its two prothallial cells, a 

 stalk cell, a tube nucleus, and two male gametes shows consider- 

 able resemblance to Ginkgo and Pinus. The inner integu- 

 ment forms a long micropylar canal, but the pollen grains are 

 drawn down into the pollen chamber formed by the disintegration 

 of the nucellar cells. Welwitschia and Gnetum are only imperfectly 

 known, but in both genera the pollen grains seem to have a prothal- 

 lial cell, a generative cell, and a tube cell. The stalk cell is elimi- 

 nated and the generative cell directly gives rise to the two male 

 gametes. 



Briefly then, although most gymnosperms possess a prothallial 

 tissue, they show a tendency toward its elimination, and in several 

 genera the male gametophyte is reduced to having a prothallial 

 or a stalk cell, a tube nucleus, and two male gametes. Only the 

 Cycadales and Ginkgoales have ciliated sperms. In the remaining 

 members the cilia have been lost and it is the pollen tube which 

 becomes the channel for the transportation of the male gametes. 

 The male gametophyte of angiosperms may be assumed to have 

 been derived from that of some gymnospermous ancestor by further 

 simplification and elimination of the single prothallial or the stalk cell. 1 



Female Gametophyte. Schnarf (1936) put forward the view that 

 the monosporic 8-nucleate embryo sue is the most primitive and 

 that all the other types have been derived from it. This idea has 

 also been favored by several other writers, the chief argument in 

 its support being that this type is the most widely distributed in 

 angiosperms and that the female gametophyte of the pteridophytes 



1 There have been occasional reports of the occurrence of a prothallial cell in 

 some angiosperms like Lilium, Eichhornia, Yucca, Sparganium, Atriplex, and 

 Stellaria (see Wulff and Maheshwari, 1938), but these are in the nature of freaks 

 and abnormalities of little or no significance. Up to the present the occurrence 

 of a prothallial cell is not known to be a regular feature in any angiosperm. 



