212 MINUTE ANATOMY OF FLOWERING PLANTS 



gonium. In the embryo sac of Gymnosperms (Conifers, etc.) a defi- 

 nite prothallial tissue is formed with rudimentary archegonia at the 

 summit. 



The pollen grain of Phanerogams corresponds to the microspore 

 of Selaginella. At the time of fertilization there are three or more 

 cells in the pollen grain and tube. These cells like those in the 

 developed microspore of Selaginella are regarded as prothallial in 

 character, two of them (those which pass through the pollen tube to 

 the embryo sac) being equivalent to antherozoids. In some Gymno- 

 sperms the fertilizing bodies from the pollen are motile, like the an- 

 therozoids of Pteridophytes. 



Thus the gametophyte of Flowering Plants is wholly within embryo 

 sac and pollen grain. In Liverworts the gametophyte (vegetative 

 thallus) is larger than the sporophyte (sporogonium). In Ferns the 

 proportions of the alternating generations are reversed, the gameto- 

 phyte being much the smaller. In Flowering Plants reduction of 

 gametophyte and increase of sporophyte have been carried to an 

 extreme. The carpels and stamens of Phanerogams are the spore- 

 bearing leaves, ovules (or their nucelli) and pollen sacs being spo- 

 rangia ; carpels and stamens are therefore often termed sporopliylls. 



XVII. THE MINUTE ANATOMY OP FLOWERING 



PLANTS 



493. Cellular structure. Attention has already been 

 called, incidentally, in several places, to the fact that plants 

 are made up of definite members of small size, called cells. 

 All new cells are formed from preexisting cells. Com- 

 monly this comes about by division : the original cell 

 divides to form two or more, each of which may increase 

 by independent growth, and in turn give rise by division 

 to new cells. The very first cell of the embryo has a 

 different origin, however. In fertilization, a nucleus from 

 the pollen tube, entering the embryo sac of the ovule, 

 fuses with a nucleus there found (see Fig. 164). As the 

 result of this union the initial cell of the new plant is 

 formed within the embryo sac. All future increase pro- 

 ceeds by division and independent growth. 



494. The cell, then, is the unit of plant structure. 

 It is the unit also of plant activity. Whatever activities 

 the plant as a whole manifests such as growth, move- 



