540 



BOTANY 



PART II 



with the outer integument is still recognisable in the ripe seed, and 

 is termed the RAPHE. Lastly the ovule itself may be curved, in which 

 case it is spoken of as CAMPYLOTROPOUS. The three types are dia- 

 grammatically represented in Fig. 508 A-C. 



As a rule only one embryo-sac is contained in an ovule. In the 

 same way as the four macrospores originate by the tetrad division in 

 the macrosporangium of Selaginella, in the macrosporangium (ovule) of 

 the Spermatophyta there is usually a single embryo-sac mother cell 

 which divides into four daughter cells ; three of these do not develop 

 further, while the fourth becomes the embryo-sac. The embryo-sac 

 of the simplest Spermatophyta also resembles the macrospore in 

 becoming filled with prothallial tissue, here termed the endosperm ; 

 one or more archegonia with large egg-cells are developed at the 

 summit of this. The fertilised ovum develops into the embryo while 



FIG. 508. A, Atropous ; B, anatropous ; C, campylotropous ovules. Diagrammatic and magnified. 

 Modified from SCHIMPER. Description in the text. 



still enclosed within the macrospore and at the expense of the parent 

 plant. When the embryo has reached a certain stage in its develop- 

 ment, which is different and characteristic in different plants, its growth 

 is arrested, and after the separation from the parent plant it under- 

 goes a period of rest. It is still surrounded by the other portions of 

 the macrosporangium, viz. the prothallium or endosperm, the nucellus 

 (if this still persists), and the seed coat formed from the integuments. 



THE COMPLETE STRUCTURE DERIVED FROM THE OVULE IS TERMED A 

 SEED, AND THE FURTHER DEVELOPMENT OF THE UNOPENED MACRO- 

 SPORANGIUM TO FORM A SEED, THE FIRST ORIGIN OF WHICH WAS SEEN 



IN THE PTERIDOSPERMEAE (p. 534), is CHARACTERISTIC OF ALL 



SEED-PLANTS OR SPERMATOPHYTA. 



The MICROSPORES of the Spermatophyta are called POLLEN GRAINS. 

 They are formed in large numbers within the MICROSPORANGIA or 

 POLLEN SACS, which are borne singly or in numbers on the MICRO- 

 SPOROPHYLLS or STAMENS. The part of the stamen which bears the 

 pollen sacs is usually clearly distinguishable and is called the ANTHER. 



