6^4 A TEXTBOOK OF THEORETICAL BOTANY 



sac is thus cut into a number of radial spaces, each containing several nuclei, 

 known as alveoli (Fig. 685). Cross walls then divide the alveoli into uni- 

 nucleate cells, completing the formation of a tissue, sometimes called the 

 endosperm, but which is really the female prothallus, contained in the 

 megaspore (Fig. 686). 



Fig. 684. — Pitius sylrestn's. Female cone at 

 the end of the first year. 



The membrane round the embryo sac corresponds to the wall of the 

 free megaspore in the Cryptogams. Although it is not so massive a structure 

 as in Selaginella or Isoetes, it is better developed in Piniis and its allies than 

 in most of the other Coniferales. As the prothallus grows the membrane 

 thickens and finally differentiates into two layers, the inner one containing 

 cellulose and the outer one completely suberized. 



The megaspore is itself contained in the nucellus, which probably 

 corresponds to a megasporangium. It should be noted that the whole of 

 this development occurs without the megaspore being detached from the 

 parent plant, so that the female prothallus is completely dependent on the 

 sporophyte for its nutrition. 



Around the prothallus the cells of the nucellus are transformed into a 

 two-celled layer of nutritive tissue, called the spongy layer, which corre- 

 sponds to a tapetum. At the micropylar end of the ovule, from one to 

 five (usually three) archegonia are produced from superficial cells of the 

 female prothallus. These archegonia are relatively simple structures, con- 

 sisting of a large venter and a short neck. An oosphere and ventral canal 

 cell are formed but there are no neck canal cells. At an early stage a layer of 

 cells round each archegonium forms a jacket layer which supplies food to 

 the oosphere. The adjacent cells of the prothallus grow faster than the 

 neck cells so that the neck lies in a depression (Fig. 687). 



The growth of the megaspore crushes the sides of the nucellus so that at 



