220 EMBRYOGENESIS IN PLANTS 



typically they fuse to form a single nucleus which may subsequently 

 divide. The uppermost cells of the gametophyte, which may be regarded 

 as being functional eggs, now begin to send out each an upwardly 

 directed tubular process. These processes penetrate the nucellar tissue 

 and eventually make contact with a pollen tube. The act of fertilisation 

 is said to consist in a movement of the female nucleus into the pollen 

 tube and a fusion there with a male gamete. The zygote thus formed 

 elongates and divides into two cells, that towards the micropyle being 

 recognised as the primary suspensor and the lower, or innermost and 

 downwardly directed cell, as the embryonic cell. There is thus no free- 

 nuclear phase. Still later, the organisation of the embryo is as is shown 

 in Fig. 52f-j. The tier labelled inner cortical ring grows basipetally 

 (relative to the embryo apex) and surrounds the primary suspensor. 

 The outer cortical ring cells then grow in a similar manner to those of 

 the inner cortical ring and in due course surround that tissue layer; 

 i.e. a cross-section would show the primary suspensor surrounded by 

 two rings of eight and sixteen cells respectively. A third and later, a 

 fourth, sheath of cells may be added to the suspensor, Fig. 52j. The 

 cap cells at the apex of the embryo undergo division but are later cast 

 off, as in Cephalotaxus. The several organs of the embryo are subse- 

 quently formed from an inner plate of meristematic cells, Fig. 52j, 

 situated below the cap. 



DISCUSSION OF EMBRYOGENESIS IN GYMNOSPERMS 



In attempting to distinguish factors which may be specially involved 

 in embryogenesis in gymnosperms, we may note that the following 

 observations and inferences have a general application. 



(i) There is evidence of basipetal physiological gradients in the 

 female gametophyte, i.e from the micropylar end (which is usually the 

 archegonial neck end) to the basal end. These gradients may be 

 important in determining the polarity and orientation of the proembryo 

 and subsequently of the embryo. 



(ii) At the time of fertilisation the ovum aflbrds evidence of proto- 

 plasmic differentiation, i.e. of structural or metabolic heterogeneity, as 

 between the upper or neck end and the basal end, the latter having the 

 greater morphogenetic potentiality. 



(iii) The extent of free nuclear division is directly related to the 

 size of the ovum : where the latter is small, as in Sequoia and some other 

 genera, there is no phase of free nuclear division. 



(iv) Both the proembryo and the embryo show polarity from the 

 outset. The proembryo is aligned along the axis of the prothallus; 

 the embryonic pole adjacent to the base of the archegonium becomes 

 the apical region, and the embryogeny is endoscopic. 



