1G4: R. BRAITHWAITE ON THE HISTOLOGY OF PLANTS. 



in which we may distinguish a nucleokis and distinct envelope. 

 As development progresses the nuclei collect round them more 

 and more protoplasm, and in the cell cavity one or more vacuoles 

 appear, filled with cell-sap. The second form is seen with one 

 daughter-cell, in the central cell of the archegonium of mosses and 

 ferns, in the resting spores of some Alg£e, as Spirogyra, CEdo- 

 goniuin, &c., in the spore-forming cells of higher cryptogams, and 

 in Pollen cells of Phsenogams. In the archegonium of mosses and 

 Hepaticse the central cell may be readily observed; granular proto- 

 plasm collects about its nucleus, and is well defined from the other 

 cell-contents ; the mother-cell, constantly enlarging, distends the 

 ventral part of the archegonium, and the young germ cell, invested 

 only by the primordial membrane, grows quickly, and at last quite 

 fills the mother-cell, the contents of which have been completely 

 consumed in the process. If impregnation does not take place 

 the contents of the germ-cell gradually become coloured brown, 

 and the membrane disappears ; but if the archegonium is impreg- 

 nated we find that resorption takes place in the wall of the mother- 

 cell turned toward the neck of the archegonium, the germ-cell 

 becomes invested by a double-contoured cellulose case, while in the 

 contents large vacuoles form, and fine internal currents may be 

 geen running between the nucleus and the parietal protoplasm of 

 the cell. 



Origin of Tetraspores and Pollen-grains. — In these the 

 primordial membrane appears to be formed round the whole con- 

 tents of the mother-cell, in which free cells form, and the cellulose 

 case is not the youngest layer of the so-called special mother- cell, 

 as SchacLt and others teach, for at a certain age the primordial 

 utricle of the mother-cell disappears, and afterwards its contents 

 are invested by one which is not in immediate contact with the 

 cellulose envelope. 



From this stage of development the young pollen cells advance 

 from the broken-up mother-cell, and appear surrounded by a sharply 

 defined nitrogenous membrane, which contracts by application of 

 iodine or syrup. To this membrane of the daughter-cell, then 

 applies itself the young primary cellulose case of the pollen grain, 

 regarded by Schacht as a thickening layer of the pollen mother- 

 cell. 



In the sporangia of some Fungi {Tuher^ Peziza, Sphceria, &c.), 

 and in Lichens, the resting spores arise as free daughter-cells, 



