Two Recent Ecological Papers. 
219 
while in the germ sporangium of the heterothallic Phycomycesnitens 
there is a partial segregation of characters, both (+) and (—) spores 
being formed. In addition to these two kinds of spores, there are 
also to be found in the Pliycomyces a few spores which give rise to 
homothallic mycelia, characterized by the occasional formation of 
zygospores and a peculiar habit of growth. The character of these 
homothallic mycelia seems unstable, for on spore-formation, both 
( + ) and (—) and homothallic spores are produced. 
One may, perhaps, suggest as a hypothesis which may possibly 
bridge over the gap between this group and the other sexual 
organisms, that the Mucorineae are derived from algal ancestors 
which exhibited a differentiation of sex, both in the gametes and 
the individuals. The groups as a whole would then appear to be 
in a transition state. Some of them, the homothallic forms, have 
lost both differentiation of the individuals and of gametes, while 
others, the heterothallic, have lost the differentiation of gametes 
while still retaining their physiological differentiation of individuals. 
The view that the group is in a transition state of reduction would 
fit in with the fact that the genus Mucor contains both homothallic 
and heterothallic forms, and also with the fact that a heterothallic 
form like Phycomyccs nitens occasionally produces homothallic 
V. H. BLACKMAN. 
strains. 
TWO RECENT ECOLOGICAL PAPERS. 
f WO papers have recently appeared, each of which makes an 
attempt to deal somewhat thoroughly with the vegetation of 
a comparatively limited area, and it is interesting to compare their 
methods and results. 
One of these is “ A Study of the Vegetation of the Mesa 
Region east of Pike’s Peak,” 1 in Colorado, and is by Mr. H. L. 
Shantz, a pupil of Professor Clements, while the other is a paper 
on the “ Ecology of Woodland Plants in the Neighbourhood of 
Huddersfield,” 2 by Dr. T. W. Woodhead, a well-known member of 
the young British school of Ecologists. 
The “ Mesa ” studied is part of the “ Great Plains ” which 
have a gradual slope eastward from the Rocky Mountains, and have 
suffered successive erosion by eastward flowing streams, refilling of 
the original valleys, and fresh erosion by the streams. The soil is 
generally gravel mixed with a certain amount of clay and humus. 
The rainfall is from 32 to 43 cm. (say 13 to 17 inches), which 
1 Botanical Gazette 42, pp. 16 and 179, July and September, 1906. 
2 Jour. Linn. Soc. Bot. 37, p. 333. Oct., 1906. 
