440 VEGETABLE PHYSIOLOGY 



in definite groups or sori. There are numerous microspores 

 in each microsporangium, which, when mature, are agglu- 

 tinated together in masses. The contents of a sporangium 

 usually exhibit two to eight of such masses, each of them 

 being known as a massula. These are set free separately. 

 A delicate skin surrounds each massula, and in some species 

 this is furnished with a number of hairs bearing barbed pro- 

 cesses or glochidia at their free ends. The mesgasporan- 

 gium, which is solitary in its sorus, bears only a single 

 megaspore. It is liberated from the sporangium, and is then 

 found to be furnished on its- lower surface with large spongy 

 bodies which are -developed from its outer coat, and which 

 serve as floats, enabling it to drift about in the water. The 

 apex of the spore bears a number of delicate filaments 

 extending between the floats. Both spores germinate after 

 liberation, each producing its appropriate gametophyte. 

 The glochidia of a massula of microspores generally catch 

 in the filaments of a megaspore, which may have arisen on 

 a different plant, and the massula thus becomes anchored 

 to the megaspore. The gametophytes are thus brought 

 together, so that the gametes can come into close propin- 

 quity to each other. 



The mechanical adaptations which have been described 

 are however not the only means we find to secure cross- 

 pollination. There are peculiarities connected with what 

 we may call the receptivity of the pistil for any particular 

 pollen. Of these the most generally occurring is dichogamy 9 

 or the maturing of the microsporophylls and the megasporo- 

 phylls of a flower at different times. Two varieties of the 

 condition are met with ; in the first, known as protandry, 

 the stamens with their pollen are mature while the stigma 

 is not sufficiently developed to be pollinated. Examples may 

 be found in the Gentianacece, Onagraceoe, Campanulacece 

 Composites, &c. In Parnassia the receptive surface of the 

 stigma is not even formed until the anthers have discharged 

 their pollen. The second condition is known as protogyny, 

 and is the converse of the first, the stigma withering before 



