58 BOTANICAL GAZETTE (JULY: 
thrive best where there are considerable quantities of organic matter.” It. 
is conceivable that Nostoc profits from intimate association with green 
plants, but to prove the parasitic nature of such association is very diff- 
cult. I could not detect that the Nostoc cells, filaments, and colonies 
within the thallus of Anthoceros appear healthier, or larger, or grow more 
rapidly, than those on the moist earth near by. The cells of this and 
many other blue-green algae are so small and the organs of the cell s0 
slightly differentiated that differences between cells are by no meats 
noticeable. From the evidence at hand it is equally unsafe to say that 
Nostoc is or is not parasitic in Anthoceros. 2 
Passing now to anatomical considerations, PRANTL’ asserts that the 
characteristic development of the thallus cavities and the formation of 
internal hairs follows the entrance of Nostoc filaments only, not of any 
other small plants. The manner of infection I will not go into, for it 
has been repeatedly described.2 The invading filament, if it survive, 
gives rise to a colony, spheroidal in form and enclosed in gelatinous mattef 
which increases with the growth of the colony. Mechanical pressure, 
increasing with the growth of the colony and with the amount of water: 
absorbed, is brought to bear against the surrounding cells of the thallus 
enlarging the cavity which the Nostoc filament entered through one of 
the slime-slits on the surface. Another effect of the increasing pressute 
is the compacting of the immediately surrounding tissue. But becaus¢ 
the Nostoc colony is not homogeneous, being in part cells and in part the 
gelatinous product of these cells, the pressure is not equal over all parts 
of the surface. The gelatinous matter between the filaments is softer and 
more readily penetrated or displaced than the filaments themselves. Ii 
small thallus cells lie opposite to and in contact with these gelatinous parts 
of a colony, they will necessarily be pushed forward by their neighbors. 
As has long been known, chains of cells, constituting the internal hairs 
above mentioned, do grow into the colonies and among the filaments of 
Nostoc. Other organisms, though they may enter the body of the liver 
wort, either do not exert any pressure at all, being smaller than the cavities 
they occupy, or form such compact masses that there is no chance for the _ 
surrounding cells to grow out as chains. : 
From this consideration of the structure and mechanics of the Nostot 
colony, we are led to see the fallacy of PRanTL’s argument that, becaust 
7 See for example KircHner, O., Schizophyceae in Engler & Prantl’s Natiir 
898. 
liche Pflanzenfamilien. I. 3248.3 
8 PRANTL, K.., loc. cit. 
9 CAMPBELL, D. H., Mosses and Ferns, Ed. 2,128. New York, 1905. 
ll 
ee 
