7H 



A TEXTBOOK OF THEORETICAL BOTANY 





Fig. 706. — Comparison of the structure of the seed in Taxus and some related 

 forms. A, Ginkgo. B, Toxospernmm (fossil). C, Vve-Torreya stage 

 (hypothetical). D, Taxus. E, Torreya. Vascular bundles transversely 

 hatched ; external outline of seed, broken lines ; inner stony integument, 

 black ; outer stony integument, stippled ; intercalated stony integument, 

 interrupted black. The diagrams illustrate varying development of the 

 inner and outer vascular systems in the integument and the intercalated 

 growth at the base of the seed which results, in Torreya, in the displacement 

 of the basal foramina to a position near the apex. {After Sahni.) 



There may be one or several archesporial cells placed hypodermally at 

 the apex of the nucellus (Fig. 707). They divide periclinally producing 

 an upper cell, i.e., the primary wall cell, and a lower cell, i.e., the primary 

 sporogenous cell. The primary wall cell divides repeatedly in a periclinal 

 direction, building up a row of cells between the primary sporogenous cell 

 and the surface of the nucellus. In this development other cells of the nucellus 

 also take part. 



The primary sporogenous cell or cells also divide repeatedly and produce 

 a compact mass of sporogenous tissue. One or more of these cells elongate 

 and become megaspore mother cells. Each megaspore mother cell divides 

 transversely into four cells, of which one, usually the lowermost, functions 

 as a megaspore and develops into an embryo sac. The others presumably 

 abort, though cases of double embryo sacs in mature ovules have been 

 recorded. 



Pollination takes place about the middle of March, while the nucellus is 

 still an homogeneous tissue. The pollen tube grows into the apex of the 

 nucellus, where it destroys a number of cells and creates a hollow. At this 

 stage it rests until the archegonia have been formed. 



