AZOLLA FILICULOIDES LAM. 
735 
always present in the leaf-cavity as well as in the neighbour- 
hood ot terminal and lateral growing points, and it is presumed 
that a symbiotic relationship exists between the alga and its 
host, but nothing is definitely known about this. The plant 
is infested with a dark-coloured Aphis, which Mr. F. Ah 
Theobald has identified as Rhopalosiphum nymphaeoe Fab., 
a common parasite on water-plantain and other aquatics. The 
dying back of the older parts, which keeps pace with apical 
growth, causes separation of the lateral branches, each of 
which, having terminal and lateral buds, becomes the nucleus 
of a number of new individuals ; and as a plant may have from 
20 to 50 primary and secondary branches and continues 
producing them indefinitely, vegetative reproduction is a potent 
factor in distribution. 
Sexual reproduction takes place by impregnation of an 
oosphere by an antherozoid. Unisexual spores are borne in 
sori produced in pairs at intervals along the stem, pendent and 
immersed. Usually male and female are associated, but two 
males or two females sometimes occur together. The male 
sorus is the larger ; it is globular, a m.m. or more in diameter, 
is easily visible to the naked eye, and has a central columella 
from which arise a score or more of stalked capsules, some of 
which are abortive. Each capsule at maturity liberates several 
(about five) microscopic bodies (the massulae of Strasburger), 
which are irregularly ovoid in shape and variable in size. 
They are composed of a colourless, tough, many-chambered 
substance armed with anchor-shaped hairs (glochidia), and 
have the (male) microspores embedded in their substance. 
The massulae have a specific gravity greater than that of water 
and sink when liberated, unless, from accidental exposure and 
desiccation, the water in the cellular tissue becomes displaced 
by air. The microspore has no independent existence ; it 
remains permanently embedded in the substance of the massula 
through which the antherozoids penetrate on their escape from 
the antheridium. 1 The female sorus is smaller — about 0’3 X 
0‘5 m.m. diameter; it is just visible to the naked eye, and 
1 Campbell. Ana. Bot., Vol. VII., p. 169. 
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