REPRODUCTION 451 
The plant in question, which is known as Azolla, is a small 
floating organism, consisting of a horizontal rhizome, some- 
times copiously branched, on which are borne numerous 
very small leaves, which are partially submerged. It bears 
spores of two kinds, both kinds produced in sporangia, 
which occur in definite groups or sori. There are numerous 
microspores in each microsporangium, which, when mature, 
are agglutinated 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 at their free ends barbed processes known as 
glochidia. The megasporangium, 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 gameto- 
phytes are thus brought together, so that the gametes can 
come into close propinquity 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 
29* 
