XIII] 



HETEROSPORY 



269 



narrow channel like a micropyle, towards which the apex of the megaspore 

 points. The distal end of the sporangium is occupied by three vacuolated 

 masses, which at maturity are stated to be filled with air, and to serve together 

 as floats for the large megaspore in the water into which it escapes. 



The whole of these remarkable structures may be referred in origin to the spore- 

 mother-cells and to the tapetum. In the microsporangium after the 64 spores are formed 

 they pass out towards the periphery of the nucleated plasmodium formed by disorganisa- 

 tion of the tapetum. This then forms a number of vacuoles, into which the spores are 

 grouped. Presently, starting from its centre, each vacuole becomes so partitioned off by 

 irregular septa that it forms an alveolated "massula," in which the spores are enmeshed 

 (Fig. 264, t). The origin of the glochidia precedes this partitioning of the vacuole. Out- 



Fig. 264. a = vacuolar membrane of a "massula" of Azolla in a young state, showing glo- 

 chidia as extensions of its surface; i^ = the glochidia more advanced, showing their in- 

 sertion upon alveolae of the massula which have now appeared; (r = transverse section of 

 a mature massula. sp.=z. microspore; /.= plasmatic residue shrunken in one of the 

 alveolae. (After Hannig.) 



growths appear like glove-fingers from the wall of the original vacuole, and they become 

 barbed at the apex (Fig. 264, a). Later they have the appearance of being seated each on 

 a single alveola, though this is not their actual origin (Fig. 264, b). Each such massula 

 escapes at maturity separately into the water in which the plant grows, with its stiff glo- 

 chidia radiating outwards. 



In the megasporangium the single large megaspore is covered by the true spore-wall, 

 outside of which is a rugged perispore, variously sculptured in different species, and some- 

 times provided with superficial whip-like threads. The whole may be held as the corre- 

 lative of one massula of the microsporangium, which is almost wholly occupied by the 

 megaspore. The three vacuolated bodies above it represent three other massulae, in which 

 the remains of the 31 aborted spores and also the tapetal nuclei may still be recognised. 

 This example will serve as illustrating the climax of development of the tapetal plasmodium. 

 It may be seen to contain chloroplasts and starch-grains, and to undergo growth and 

 development after the identity of its constituent cells has been lost. 



