CHAPTER 13 

 THEORETICAL CONCLUSIONS 



The phylogeny and interrelationships of angiosperms present 

 problems which have baffled botanists for many years. As shown 

 in Chap. 11, embryology has been of appreciable help in reorienting 

 our ideas on the interrelationships of several doubtful families and 

 genera. In the present chapter we shall consider some problems 

 of wider interest concerning the origin and homologies of the male 

 and female gametophytes, endosperm, etc. 



Male Gametophyte. In discussing the homologies of the male 

 gametophyte of angiosperms, we must naturally turn to the condi- 

 tion in gymnosperms. The available evidence suggests that in the 

 fossil gymnosperms the pollen grains were multicellular structures 

 containing both prothallial and spermatogenous cells. Probably 

 there were no pollen tubes and the sperms made their way directly 

 to the archegonia. Swimming sperms are found even in some mod- 

 ern representatives of the group, viz., the living cycads and Ginkgo, 

 but in addition a pollen tube is also present. It is interesting to 

 note that the tube originates from the upper end of the pollen grain 

 and grows laterally into the nucellar tissues, acting as a haustorial 

 and not a sperm-carrying structure. The basal end of the pollen 

 grain hangs free in a cavity, which may be said to be composed 

 partly of the pollen chamber and partly of the archegonial cham- 

 ber. There are present, beside the two sperms, the prothallial 

 cells (one in the Cycadales and two in the Ginkgoales), the stalk 

 cell, and the tube nucleus. In Microcycas there are 16 to 22 

 sperms, which should probably be considered a primitive feature. 



The Coniferales differ in two important respects: (1) the sperms 

 do not possess any cilia, and (2) the pollen tube does not arise from 

 the upper end of the pollen grain but from its lower end, penetrat- 

 ing through the nucellus and discharging its contents into 

 the archegonium. The contents of the pollen grain and the tube 

 vary in different genera. In Araucaria, Podocarpus, Dacrydium, 

 and Phyllocladus there are several prothallial cells; in Pinus and 



411 



