LYCOPODIALES 



323 



It will be noted that there is no interruption of the development by 

 a period of rest, as in the matured seeds of ^'lowering Plants. 



The life-cycle of Selaginella, though it does not involve the formation 

 of a seed, or include any dormant period, corresponds in its essentials 

 to that in Flowering Plants, In both cases the plant itself is the 

 diploid, or sporophyte generation. The events may be compared, 

 starting from the production of the spore. The fact that the pollen- 

 sac and ovule of seed-bearing plants differ in form and position from 

 the microsporangia and megasporangia of Selaginella does not prevent 

 their comparison respectively with them. In the pollen-sac, as in 

 the microsporangium of Selaginella, a tetrad-division leads to the 



Fig. 265 bis. 



Emhiyos ot Selaginella dentictdata, aiterBruchmaim. £/ = suspensor. iCf/=root. 

 /= foot. &6= basal wall. /:— hypocotyl. 5= apex. ^, A, ^ cotyledons, ^^ligule, 

 (After Bruchmann.) 



formation of four spores from each tetrad. The microspores in each 

 case are shed on rupture of the sporangial wall. In the ovule, as in 

 the megasporangium of Selaginella a tetrad-division leads up to the 

 formation of the megaspore. In both cases only one spore-mother- 

 cell as a rule undergoes the division, but in both cases there may 

 occasionally be more than one. In Seed-Plants usually only one 

 megaspore, or embryo-sac, is matured. In Selaginella commonly four, 

 and occasionally more ; but sometimes two, or even only one. Thus 

 in point of origin, in manner of production and sometimes in 

 number, the megaspore corresponds to the embryo-sac. The differ- 

 ence lies in the fact that in Selaginella the megaspore, with its thick 

 protective wall, is shed on rupture of the megasporangium ; in Seed- 

 Plants the megaspore, or embryo-sac, is retained within the nticellus. 

 The embedded sac is thus able to obtain continued nourishment. 

 The biological advantage thus gained offers a ready explanation of 



