7o6 



A TEXTBOOK OF THEORETICAL BOTANY 



The Female Gametophyte. 



The nucleus of the megaspore divides to form 256 free nuclei, arranged 

 round the periphery of the rapidly enlarging cell. The formation of the 

 prothallial tissue follows by centripetal alveolation, very much as in Pinus, 

 but the completed prothallus has a longitudinal strand of elongated cells, 

 suggesting a conducting function, which is peculiar to Taxus. Although the 

 prothallus cells are primarily uninucleate there is a multiplication of their 

 nuclei during July so that eventually each cell may contain up to sixteen 

 nuclei, except in the central strand of cells where there are rarely more than 

 three or four. Finally the nuclei in each cell degenerate and fuse, first into 

 three or four masses and finally into one large irregular mass. 



The number of archegonia varies, usually between five and eight. They 

 originate from cells two or three deep from the upper surface of the pro- 

 thallus and may be united in a close group or may be variously distributed 



Fig. 708. — Taxus baccata. A and B, Development of 

 the embryo showing elongated suspensor cells 

 and embryonal group. C and D show the 

 organization of a pyramidal apical cell. {After 

 Strasburger.) 



in the tissues. Often the prothallus tissue proliferates around the archegonia, 

 rising beyond them so that they are left at the bottom of deep pits. Each 

 archegonium is very simple. There are only four neck cells and no ventral 

 canal cell has been recorded. 



Several archegonia may be fertilized, but only one embryo normally 

 develops to completion. The first stage is free nuclear division in the oospore, 

 up to thirty-two nuclei being formed before they migrate to the base of the 

 oospore, when wall formation divides them into separate cells, which are 

 arranged in three layers. The lowest layer forms the embryo and the middle 

 layer the suspensors, very much as in Pinus (Fig. 708). The mature embryo 

 is straight and has two cotyledons. It consumes all the endosperm (prothallus) 

 during its maturation. 



The seedling has two linear cotyledons which remain until the third year. 

 They are somewhat larger than the foliage leaves which follow them, but are 

 otherwise similar and have a single vascular bundle. The primary root is 

 diarch and lateral roots soon begin to appear. 



