1904] CURRENT LITERATURE 155 
case the essential feature is a sensitive layer of protoplasm and an apparatus 
which concentrates the light upon this plasma, of which certain regions are either 
more or less strongly illuminated when the organ is out of its normal relation to 
the incident light. Two types are distinguished: those in which the outer face 
of the epidermal cell is convex, and those in which the inner wall is convex toward 
the mesophyll or is the frustum of a cone. The hypothesis is not supported by 
any experimental evidence, but is constructed merely from anatomical observa- 
tions and a priori reasoning. (See also p. 157.)—C. R. B. 
Wacer?s has studied the nucleolus during nuclear division in the root of 
Phaseolus. After a summary of the extensive literature and a description of 
methods, the subject is presented under the following heads: the resting nucleus, 
structure of the nucleolus, changes in the nucleolus during the prophase, and 
reconstitution of the daughter-nuclei. The main conclusions are that the nucleo- 
lus simply forms a part of the nuclear network, in which chromatin or chromatin- 
substance may be stored, and therefore is not an independent organ of the nucleus; 
that it is concerned in the formation of the chromosomes, and possibly also in 
the production of the spindle, and that a portion of it may in some cases 
extruded into the cytoplasm and there disappear; that in the reconstruction of 
the daughter-nuclei the chromosomes unite together in a more or less irregular 
mass or thick thread, out of which is evolved the nucleolus and nuclear network, 
the major part of the chromatin passing ultimately into the nucleolus, except 
in cases where division again immediately takes place. Attention is called to 
the fact that if these conclusions are correct, the part played by the chromosomes 
in heredity will need revision, and that the nucleolus as well as the chromosomes 
will have to be taken into account.—J. M. C. 
SaALmon?° has published the results of experiments with the so-called ‘‘bio- 
logic forms” of the Erysiphaceae, that is, races of individuals morphologically 
identical, but differing physiologically in possessing distinctive and sharply 
defined powers of infection. This specialization of parasitism has been found 
to be associated with both conidia and ascospores. The present experiments 
show that the restriction in power of infection characteristic of “biologic forms” 
breaks down if the vitality of the leaf is interfered with in certain ways, as b 
wounding. It was found also that conidia produced on a wounded leaf that was 
normally immune to such attack would infect uninjured leaves of the plant in 
question; by means of this “bridge”? passing from one host plant to another. 
Injuries to leaves in nature, resulting in such bridging, were observed to be made 
by the ‘green fly” (Aphis). Therefore, in the evolution of “biologic forms” 
two sets of factors are at work: one, called “specializing factors,” tending to 
Specialize parasitism and deriving from a single morphological species a number 
*s WaGER, Haron, The nucleolus and nuclear division in the root apex of Phase- 
olus. Annals of Botany 18:29-55. pl. 5. 1904- 
: SALMON, Ernest S., Cultural experiments with “biologic forms” of the Ery- 
Siphaceae. Phil. Trans. Roy. Soc. London B. 197: 107-122. 1904- 
