242 



THIRD GROUP. — VASCULAR CRYPTOGAMS. 



cells, each of which produces four spores formed and tetrahedrally disposed in the 

 usual manner ; during this process the tapetal cells too are gradually disorganised, 

 and a granular protoplasm then fills the space in the sporangium between the 

 isolated mother-cells and the spore-tetrads, and is subsequently used to form 

 the peculiar episporia or gelatinous envelopes of the spores. Up to this point 

 the course of development in the two kinds of sporangia is the same, but beyond 

 this point it differs. In the microsporangia all the spores in the sixteen tetrads 



arrive at maturity ; each young spore in the 

 mother-cell, which is now divided into four 

 compartments, assumes its permanent mem- 

 brane, and then the walls of the compartments 

 in the mother-cell are dissolved. In the ma- 

 crosporangia, on the contrary, one of the young 

 spores in each of the sixteen tetrads grows at 

 first more vigorously than the other three, but 

 ultimately all the tetrads except one waste 

 away, and in this one the preferred cell, the 

 future macrospore, grows very vigorously, while 

 the other three languish. Figs. 198 and 199 

 show the development of the macrospore of 

 Pilularia globuli/era, from drawings made by 

 Sachs in 1866; they show the young ma- 

 crospore (/, //, ///) still in connection with 

 its three sister-cells, which are invested with 

 the substance of the cell-walls of the four 

 compartments of the mother-cell already con- 

 verted into mucilage (/); the four cells adhere 

 by their spike-like projections, that of the 

 macrospore being the most strongly developed. 

 After a while the macrospore is seen to have 

 increased greatly in size; the sister-cells which 

 have dwindled away hang to it at its side 

 (Fig. 199 x); its firm membrane has become 

 brown, and is invested with a covering of 

 mucilage (Fig. 198 IV, b), which often ap- 

 pears to be folded. It subsequently forms a 

 papilla above the apex, which shrivels w-hen the 

 spore is ripe (Fig. 199//). Later a layer of a white substance of marked prismatic 

 structure is formed on the mucilaginous covering (Fig. 199 c), and this is afterwards 

 overlaid by a still thicker covering of a less evidently organised substance. Both 

 envelopes leave the apex uncovered, and thus form the funnel, through which the 

 spermatozoids enter (Fig. 186). The macrospores of Marsilia have a similar 

 episporium, but its development is not satisfactorily explained in the descriptions 

 which we at present possess \ 



Fig. 197. Development of the sporangium of Pilu- 

 laria globuli/ira, all the figures being in optical longi- 

 tudinal section ; c archesporium, stn spore-niother-cells. 



* Russow, loc. cit. p. 228. 



