SUBKINGDOM SPERMATOPHYTA 



325 



the Pteridophytes, and in exceptional cases e.g. Cycas, Zamia 

 large ciliated spermatozoids develop from them. The pollen-spore, 

 when ripe, often has the antheridial cell separated from the sterile 

 cell, and when it germinates, which it will readily do in a 10 to 15 

 per cent sugar solution, sends out a germ-tube, or pollen-tube, 

 through a rupture in the outer spore-coat. The division of the 

 generative nucleus commonly takes place within the pollen-tube. 



Female Gametophyte. Among the lower Spermatophytes the female 

 gametophyte closely resembles that of the heterosporous Pterido- 

 phytes, especially Isoetes and Selaginella. Archegonia of the same 

 type are developed, and the gametophyte resembles much more that 

 of the Pteridophytes than that of the higher Spermatophytes. In 

 the latter the gametophyte becomes excessively reduced, and the 

 homologies of the structures found in the fully developed macrospore, 

 or embryo-sac, are not entirely 

 clear. 



The Embryo 



Usually, each fertilized egg-cell 

 gives rise to a single embryo, 

 either by direct cell-division, or 

 after several free nuclei have been 

 formed. In some Coniferse, how- 

 ever, each egg gives rise to four 

 embryos. A suspensor, similar 

 to that in the embryo of the 

 Lycopodiales, is found in most 

 Spermatophytes. 



Classification of Spermatophytes 



X 



1 



P i w m 



FIG. 288. Taxodium distichum. Trans- 

 verse section of the stem at the be- 

 ginning of the growing season ; cam, 

 cambium; x, xylem ; ph, phloem; 

 m, medullary ray ; the xylem is com- 

 posed of tracheids with bordered pits, 

 p, on their radial walls (x 400). 



Two classes of Spermatophytes 

 are recognized, Gymnosperms and 

 Angiosperms ; but whether these are directly related may be ques- 

 tioned. In the former the ovules, or macrosporangia, are exposed 

 upon open sporophylls, as they are in the Pteridophytes ; in the 

 Angiosperms the ovules are always borne in a closed cavity, the 

 ovary, formed from the base of the carpel (sporophyll) or from 

 the coherent bases of two or more carpels. A more important dis- 

 tinction is the very much reduced female gametophyte of the 

 Angiosperms. 



CLASS I. GYMNOSPERMS 



The Gymnosperms are the oldest types of seed-bearing plants, and 

 in many respects, especially in the character of the gametophyte, 



