SECT. II PHANEROGAMIA 477 



the Phanerogams 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 formation of 

 the piiOTHALLiUM (or endosperm) and of the arghegonia (or egg-cells) 

 differs in the several classes of Phanerogams. The fertilised ovum 

 grows into the embryo while still enclosed within the macrospore and 

 at the expense of the parent plant. AVhen the embryo has reached a 

 certain stage in its development, which is cliflferent and characteristic 

 in diff"erent plants, its growth is arrested, and after the separation 

 from the parent plant it undergoes 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 macrosporangium to form a seed 

 IS CHARACTERISTIC OF ALL PHANEROGAMS. As seed-plants or Sperma- 

 phyta the latter may be contrasted with the seedless Cryptogams. 



An account of the male sexual generation of the Phanerogams 

 must now be given for comparison with the above short sketch of 

 the development of the female generation. 



The MICROSPORES of the Spermaphyta 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. 



The development of the pollen-sac (Fig. 423) commences with 

 divisions parallel to the surface taking place in cells of the hypodermal 

 layer ; this separates the cells of the primary archesporium from an 

 outer layer of cells. The latter give rise to three layers of cells the 

 outermost of which is the fibrous layer, the innermost the tapetum, 

 while the intervening layer is later crushed. The archesporium after 

 undergoing a number of divisions forms the pollen-mother -cells, each 

 of which divides as in Pteridophytes into four daughter-cells (cf. Fig. 

 102). These are the pollen-grains, and are spherical or ellipsoidal in 

 shape and provided with a cell wall ; an external cutinised layer (the 

 exine), and an inner cellulose layer, rich in pectic substances (the 

 intine), can be distinguished in the wall. 



While the male sexual cells of all archegoniate plants are depend- 

 ent on water for their conveyance to the female organs, the transport 

 of the pollen-grains to the egg-cells is brought about in Seed-plants 

 by means of the wind or by animals. However far the reduction of 

 the male prothallium has proceeded — and even in the case of the 

 heterosporous Pteridophyta only a single sterile cell was present — two 

 constituent parts are always distinguishable in the germinating pollen- 

 grain ; these are a VEGETATIVE CELL which grows out as the pollen- 

 tube, and an ANTHERIDIAL MOTHER-CELL which xdtimately gives rise 



