'242 



CRYPTOGAMS AND PHANEROGAMS. 



sacs are formed (Fig. 248 C-E), with this difference only, that 

 while a great many cells may be distinguished in each pollen- 

 sac, which forms pollen-grains by tetrad-division, only a few are 

 found in the ovule, and all these moreover are suppressed, with one 

 siui/le exception which developes into the macrospore (embryo- 

 sac) without undergoing a division into tetrads. The wall of the 

 embryo-sac, in the Grymnosperms, may be thick and divided into 

 two layers and partly cuticularized, as in the spores of the Cryp- 

 togams which are to be set free. In the Angiosperms, on the other 

 hand, the wall is extremely thin. 



The pollen-sac thus stands in the same relation to the nucellus 

 as the micros porangium does to the macrosporangium : in the 

 pollen-sacs and microsporangia a number of spores arise by the 

 tetrad-division of several mother-cells ; in the nucellus and macro- 

 sporangium, a reduction of the cells already formed takes place to 

 such an extent that the number of macrospores becomes one 

 (Salvinia, Harsilin, Phanerogams) or four (Selaginella), or rarely a 

 large number as in Isolates. 



In the Ferns, as stated on page 210, etc., indnsia covering the 

 sori very often occur. Horsetails and Club-Mosses have no 

 indusium; but in all Phanerogams cupular or sac-like structures 

 (integuments') are found which envelop the nucellus. These de- 

 velope from the upper end of the funicle (ii and ie, in Fig. 248 ; 

 y and i, in Fig. 249) and enclose the nucellus on all sides as a 



sac, leaving only a 

 small channel at the 

 apex of the nucellus 

 the m icropyle 

 (Fig. 249) through 

 which the pollen- 

 tube proceeds to the 



FIG. 219.-Various forms of ovules: .4 an erect ovule embryo - sac. The 



(orlhotropous) ; -B reversed (anatropous); C curved (campt/7o- ovules of the Grym- 



',-. T oi(s) : fc the nueellus (shaded in all the figures) ; s the em- .ho, m lu 



bryo-sac ; ,-h the base of the ovule (chalaza) ; i, and i the y 



external and internal integuments, the dotted line denotes One integument 



bhe place where the scar (hilum) will form when the seed /T7- , 9^1 Oft t OtfQ 



is detached from the funicle. * lgS ' ^^ ^^ ^ J ' 



274) and the same 



i> the case with the majority of the Sympetalae and a few Cho- 

 ripftalsv; but the Monocotyledons and most of the Choripetalte 

 have two integuments (Fig. 249). 



In shape the integuments resemble very closely the cupular 



