300 



FOURTH GROUP. 



corpuscula\ The processes in the macrospore or embryo-sac of the Monocotyledons 

 and Dicotyledons are not so easy to understand. In their case three cells are formed 

 at each of the two extremities of the embryo-sac ; one of the two groups of cells is 

 known as the egg-apparatus, and may be regarded as three archegonia reduced each 

 to one cell (a similar reduction is found among the Gymnosperms in the gnetaceous 

 genus Welwitschia), while the other three cells are called the antipodal cells and are to 

 be considered as a rudimentary prothallium. Here too a tissue, the endosperm, is 

 formed in and fills the embryo-sac after fertilisation, but the cells of the rudimentary 

 prothallium do not take part in its formation ; this commences with the division of 

 the nucleus of the embryo-sac, which is still present along with the six cells. We 

 must not therefore consider the endosperm of the Angiosperms as equivalent to the 

 endosperm of the Gymnosperms, which, as has been said, is simply the tissue of the 

 prothallium in the macrospore, whereas the endosperm of the Angiosperms, as 

 compared with the Vascular Cryptogams, is probably to be regarded as a new 

 formation. 



The macrosporangium of the Seed-plants is termed the ovule. The arche- 

 sporium is formed in it exactly in the way described above, for instance in the case 

 of Isoetes, but the sporogenous tissue is usually much reduced, being limited to a few 

 cells; in the Cycadeae and in some Coniferae it developes into a tolerably large 

 mass of cells ; and the macrospore is not produced by the division of a mother-cell 

 into four parts, but from a group of a few cells formed by division of the archesporium, 

 one of which by its growth displaces the others and becomes the macrospore. The 

 macrosporangium of the Seed-plants is also distinguished from that of the Vascular 

 Cryptogams by being usually enclosed in one or two envelopes, the mtegutneuts, which 

 project above its apex and from which the seed-coat is afterwards chiefly formed. 

 These integuments differ from the envelopes (indusia) of the macrosporangia in the 

 Vascular Cryptogams in being formations from the base of the young macro- 

 sporangium itself, and not luxuriant outgrowths of the leaves which bear the sporangia, 

 as in the Ferns and Isoeteae. It is only in the microsporangia of certain of the 

 Coniferae, namely, the Cupressineae, that we find formations of the nature of an 

 indusium. The part of the macrosporangium which is enclosed by the integuments 

 is termed the nucellus. 



The microspores of the Seed-plants bear the name of pollen-grains. They 

 too, like the microspores of the Vascular Cryptogams, produce a rudimentary male 

 prothallium, which in the Angiosperms is usually represented by only one cell, and 

 this cell is not even separated oif by a firm cellulose-membrane -. 



The pollen-grains, like all microspores, contain the male or fertilismg principle, 

 which passing into the oosphere stimulates it to the formation of the embryo ; but 

 there is a great difference in the way in which the transmission of the fertilising 

 substance is effected. In the Vascular Cryptogams this substance is a modle 



' [As Vines points out (Sachs, Lehrb., Engl, transl., 2nd ed. p. 499), archegonium and corpus- 

 culum are not exactly synonymous, since the latter, properly speaking, is only equivalent to central 

 cell of the archegonium. 



- Slrasburger (Neue Unters. liber d. Befruchtungsvorgang b. d. Phanerog. 1884) dissents from 

 the view that the cell or cells so cut off are homologous with the prothallium in the Vascular 

 Cryptogams.] 



