REPRODUCTION 441 
very small leaves, which are partially submerged. It bears 
two kinds of spore, each produced in sporangia, which occur 
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 asa 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 megaspor- 
angium, which is solitary in its sorus, bears only a single 
megaspore. Itis 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, 
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 Gentianacee, Onagracee, Campanulacee, 
Composite, &e. In Parnassia the receptive surface of the 
stigma is not even formed until the anthers have discharged 
