1 82 Campbell . — On the Development of 
The mesophyll of the cotyledons and the leaves immediately 
following does not show the peculiar elongated cells found in 
the later leaves, and these first appear in about the fourth 
leaf, but are not as well developed as in the later ones. In 
all of the leaves, however, with the exception of the cotyledon, 
the peculiar cavity, filled with a colony of Anabaena , is to be 
seen. 
The Anabae7ia begins to grow almost as soon as the first 
divisions in the embryo are completed. If a young embryo 
is dissected out, it will be found that in the space between the 
cotyledon and the stem, that a number of very short Anabaena - 
filaments are present. As the embryo pushes up through the 
space between the archegonium and the indusium, the Ana- 
baena- cells collected there are carried up with it, and then 
begin to grow. They assume the blue-green colour of the 
active cells, elongate and divide rapidly by a series of trans- 
verse walls into short filaments that ' at first look like an 
Oscillai'ia . Very soon, however, the cells round off. hetero- 
cysts are formed, and the typical form of the ordinary filaments 
is attained. Some of these remain tangled about the growing 
point of the stem, while others creep into the cavities at the 
base of the leaves. 
No branches are formed in the young plant before about 
eight or ten leaves are produced. Whether the position of 
the first branch is constant, I cannot say, as the point was not 
critically examined. 
Conclusion. 
A comparison of the development of Azolla with other 
forms does not show a very close resemblance to any one, and 
indicates a somewhat isolated position for the genus. Its 
nearest ally is unquestionably Salvinia , with which it agrees 
in the general plan of its growth, the two-sided apical cell of 
the stem, and especially the development of the sporocarp. 
However, as Strasburger has shown, except the first divisions 
of the apical cell, the resemblance is not very close, and in the 
decidedly apical growth of the leaves and the absence of 
