514 



BOTANY 



PART II 



when mature, a thin wall of one cell-layer, but no annulus (B, D). The MICRO- 

 SPORANGIA enclose a large number of microspores, which, as a result of their 

 development in tetrads from the mother-cells, are disposed in groups of four (C), 

 and embedded in a hardened frothy mass filling the cavity of the sporangium. 

 This frothy interstitial substance is derived from the tapetal cells, which gradually 

 lose their individuality and wander in between the spore-mother-cells. 



The microspores germinate within the microsporangium, which does not open ; 

 each germinating microspore puts out a short tubular male prothallium, which 

 pierces the sporangial wall. Two antheridia are developed in this by successive 

 divisions (Fig. 480). Each antheridium produces four spermatozoids, which are 

 set free by the rupture of the cell walls. Although the whole male prothallium is 

 thus greatly reduced, it nevertheless exhibits in its structure a resemblance 

 to the prothallia of the Filices. 



The MACROSPORANGIA are larger than the microsporangia, but their walls 



ma 



mi 



FIG. 479. Salvinia natans. A, Three sporocarps in median longitudinal section ; ma, macro- 

 sporocarp ; mi, microsporocarp ( x 8) ; B, a microsporangium ( x 55) ; C, portion of the contents 

 of a microsporangium, showing four microspores embedded in the frothy interstitial substance 

 ( x 250) ; D, a macrosporangium and macrospore in median longitudinal section ( x 55). 

 (After STBASBURGEK.) 



consist similarly of one cell-layer (Fig. 479 D). Each macrosporangium produces 

 only a single large macrospore, which develops at the expense of the 32 

 spores originally formed. The macrospore is densely filled with large angular 

 proteid grains, oil globules, and starch grains ; at its apex the protoplasm is 

 denser and contains the nucleus ; the membrane of the spore is covered by a dense 

 brown exospore, which in turn is enclosed in a thick frothy envelope, the perispore, 

 investing the whole spore and corresponding to the interstitial substance of the 

 microspores, and like this formed from the dissolution of the tapetal cells. The 

 macrospore remains within the sporangium, which is eventually set free from the 

 mother plant and floats on the surface of the water. On the germination of the 

 macrospore, a small-celled female prothallium is formed by the division of the 

 denser protoplasm at the apex, while the large underlying cell does not take part 

 in the division, but from its reserve material provides the developing prothallium 

 with nourishment. The spore wall splits into three valves, the sporangial wall is 

 ruptured, and the green prothallium protrudes as a small saddle - shaped body. 

 On it three to five archegonia are produced, but only the fertilised egg-cell of one 

 of them develops into an embryo, the foot of which remains for a time sunk in 



