212 illNUTE ANATOMY OF FLOWERiyO PLANTS 



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

 iiite protliallial tissue is formed witli rudimeiitarj' arclieguuia at the 

 summit. 



The pollen grain of Phanerogams corresponds to tlie rnicrosjiore 

 of Selaginella. At the time of fei'tilization there are three or more 

 cells in the p'olleii gi'ain and tube. These cells — like those in the 

 developed mici'ospore of Selaginella — are regarded as prothallial i}i 

 character, two of them (those which pass through the puUen tul.ie to 

 the embi'vo sac) being equivalent to antherozoids. In some Gyuuio- 

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

 therozoids of Pteridophytes. 



Thus the ganietophyte of Flowering Plants is wholly w itliin embryo 

 sac and p'ollen grain. In Liverworts the gametopiliyte (vegetative 

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

 proportions of the alternating generations are reversed, the ganieto- 

 phyte being much the smaller. In Flowering Plants reduction of 

 gametophyte and inci-ease of sporophyte have been carried to an 

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

 bearing leaves, ovides (or their uucelli) and pollen sacs being spo- 

 rangia; carpels and stamens are therefore often termed sporophi/Us. 



XVII. THE MINUTE ANATOMY OP FLOVTBRING 



PLANTS 



493. Cellular structure. — Attention ha.s already been 

 called, iucidentally, 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 preuxistino- cells. Com- 

 monly this comes about by division : the original cell 

 di\"ides to form two or more, each of which may increase 

 by independent grow lli, and in turn give rise by division 

 to new cells. The very first cell of the embr3"o has a 

 different origin, however. In fertilization, a nucleus from 

 the pollen tube, entering the eml)r3-o sac of the ovide, 

 fuses witli a nucleus there found (see Fig. 1(J4). As the 

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

 formed witliin tlie embryo sac. All future increase pro- 

 ceeds Ijy division and independent growtli. 



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

 It is the itnit also of plant activity. Whatever acti\dties 

 the plant as a whole manifests = — such as giowtli, move- 



