SUBKINGDOM SPERMATOPHYTA 323 



highly probable that seeds have arisen independently in different 

 groups of Pteridophytes, it is by 110 means certain that all Sperma- 

 tophytes are derived from a common stock. 



The protection of the macrospore with the enclosed garnetophyte, 

 within the macrosporangium, or ovule, is apparently very advan- 

 tageous, as the Spermatophytes are the plants which have succeeded 

 best in adjusting themselves to the conditions now prevailing upon 

 the earth. 



Fertilization in Spermatophytes 



The position of the female gametophyte in the Spermatophytes 

 necessitates a different method of fertilization, and in all of these 

 the germinating microspore (pollen-spore) produces a long tube, the 

 pollen-tube, into which pass the male generative cells, and these are 

 thus conveyed to the egg-cell. Among the lowest of the Seed-plants, 

 i.e. Cycads and Ginkgo, large ciliated spermatozoids are developed 

 within the pollen-tube, but in all other cases the male cells are des- 

 titute of cilia, and the pollen-tube discharges the generative nuclei 

 directly into the egg-cell, or into a neighboring cell (Synergid), 

 through which it is conveyed to the egg. In case the pollen-tube has 

 to traverse the tissues of the pistil or ovule, it grows through them 

 very much as the hypha of a Fungus would do, undoubtedly grow- 

 ing at the expense of the cells among which it passes. 



The Flower 



The sporophylls of the Spermatophytes are usually aggregated, 

 and form the Flower of these plants, which are often, therefore, called 

 the Flowering-plants. It must be remembered, however, that the 

 cone of sporophylls in Equisetum or Selaginella might, with equal 

 propriety, be considered a flower, and it is the seed and pollen-tube, 

 and not the flower, which must be considered the distinctive features 

 of this group. 



The Spores 



Microspore. The microspores of the Spermatophytes, or pollen- 

 spores, as they are more commonly termed, agree in all respects with 

 the spores of the Archegoniates. They always arise from the divi- 

 sion of a sporogenous cell into four spores, and these in their struc- 

 ture agree exactly with those of the typical Archegoniates. Like 

 them, also, they are always discharged from the sporangium, and 

 complete their germination away from it. 



Macrospore. The macrospores agree in their early development 

 with those of the Pteridophytes, but a true tetrad division is usually 

 absent, and only in rare cases does the spore develop an outer thick- 



