Form of Hormidium flaccidtini , A. Braun . 515 
Rarely, a cell becomes abnormally wide and subsequently divides by 
a longitudinal wall ; two or three adjacent cells may behave in this manner 
(Fig. 1, D and E). A filament has never been observed more than two cells 
wide in the abnormal part, but the longitudinal walls may arise somewhat 
irregularly. 
It will be seen from the above that of the three forms of Hormidium 
flaccidum enumerated by Heering 1 in his recent review, the Alga under 
consideration approaches the form aquatica , recorded as growing in stagnant 
water. It differs from the latter only in having a considerable proportion 
of short cells, of less length than breadth, and in habitat. 
2. Mode of Growth. 
At Woodford, the Alga occurs in isolated patches, colonizing some 
large, bare tracts which interrupt the continuity of the grassland. These 
tracts were evidently overgrown by grass at some previous time, since a 
layer of grass roots and rhizomes is habitually present beneath the Hor- 
midium layer. The whole area is low-lying, there being a pond and system 
of small streams in the immediate vicinity, which, no doubt, help to keep the 
soil relatively moist. The matting of grass remains, referred to above, also 
tends to keep the surface layer of Hormidium damp by preventing water 
from quickly penetrating the soil. The latter is relatively light and remains 
continuously damp during the winter, but becomes very dry during the hot 
summer months. 
There is a certain competition between the grass and the Hormidium , 
especially at the junction of the two zones. The grass cannot endure for 
any considerable period the intense dry heat which occurs from time to time 
in midsummer, and soon withers. The Alga, however, is able to resist 
desiccation for a longer period, and on the return of wet weather resumes 
its normal growth. It then tends to occupy former grass areas. At the 
approach of spring, however, the grass fruits, lying in the meshes of the 
tangle of old stems and roots, germinate, and the new plants overgrow 
the surrounding Hormidium , which tends to die away. Thus the edge of 
the grassland once again establishes itself. 
The Alga grows as a thin layer scarcely 1 mm. thick over the surface 
of the ground. It is not definitely associated with the grass remains below, 
though occasional filaments grow downwards and then often coil round the 
latter . 2 Most of the threads, however, lie horizontally and interlace, pro- 
ducing a weft. The superficial filaments are much richer in the granules 
1 W. Heering : Ulotrichales, Microsporales, Oedogoniales, in A. Pascher, Die Susswasserflora 
Deutschlands, Osterreichs und der Schweiz, Heft vi, 1914, p. 46. 
2 It may be noted that the filaments of another form of Hormidium flaccidum , found on the thin 
soil covering some stones in Victoria Park, appeared to bend down in a knee-like manner here and 
there into the soil; the cells at the bends had usually lost their contents. 
