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 

 single exception which developes into the macrospore (embryo- 

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

 embryo-sac, in the Gymnosperms, 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 nucellu& 

 as the microsporangium 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, Marsilia, Phanerogams) or four (Selaginella), or rarely a 

 large number as in Isoetes. 



In the Ferns, as stated on page 210, etc., indusia 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 micropyle 

 (Fig. 249) through 

 which the pollen- 

 tube proceeds to the 



A an erect ovule em ' :) I'JO - Sac. 

 (orlhotropous) ; B reversed (anah-opous); C curved (campylo- ovules of the Gym- 

 tropous) : fc the nucellus (shaded in all the figures) ; s the em- nosperms have On! V 



f ? 



integument 



251 264 269 



274) and the same 

 is the case with the majority of the SympetalaB and a few Cho- 

 ripetal^; but the Monocotyledons and most of the Choripetalfe- 

 have two integuments (Fig. 249). 



In shape the integuments resemble very closely the cupular 



FIG. 2 19. -Various forms of ovules 



bryo-sac ; c?i the base of the ovule (chalaza) ; y and i the 

 external and internal integuments, the dotted line denotes One 

 the place where the scar (hilum) will form when the seed 

 is detached from the f uuicle. 



