734 
AZOLLA FILICULOIDES LAM. 
V. 
AZOLLA FILICULOIDES LAM. 
By W. H. Burrell, F.L.S. 
Read October 27th, 1913, and Supplemented. 
The genus Azolla is classified under the Archegoniatae, 
showing affinity with ferns in having the sporophyte as the 
dominant stage in the alternation of generations. It differs 
from them in yielding two kinds of spores, producing antheridia 
and archegonia on separate prothallia, whereas ferns are homo- 
sporous, antheridia and archegonia being borne by the same 
prothallium. Azolla was formerly included in the N.O. 
Marsiliaceae, its segregation with Salvinia in a new order, 
Salviniaceae, depending on the separation of male and female 
spores in distinct soii, the old order Marsiliaceae being reserved 
for the two genera, Marsilia and Pilularia, whose spores of 
two kinds are enclosed in one sporangial chamber. As it 
occurs in Norfolk, the mature plant measures about two 
inches in length and one inch in width, and has dark-coloured 
unbranched roots descending from the stem at frequent 
intervals. Stems up to 90 m.m. (nearly four inches) have 
been measured, with roots 80 m.m. long. The prevailing colour 
in summer is light green, with a good deal of pink on the 
ventral leaf segments ; in autumn the colour changes to brick- 
red, the green re-asserting itself in June. The leaves are 
placed alternately in two lateral rows, each leaf being cut 
almost to the base into a dorsal and ventral segment. The 
upper or dorsal segment contains a relatively large cavity, 
which opens by a pore on its lower surface. The remarkable 
buoyancy of the plant is due to air held in these leaf cavities 
and in the lax tissue which forms the leaf base. Chains of 
cells of a minute blue-green alga ( Anabcena Azollce) are 
