1906] CURRENT LITERATURE 301 
NOTES FOR STUDENTS. 
What is a species?—The many discussions as to what is a species have 
resulted in a general appreciation of the facts that species are not all of equal 
rank, that they are distinguished by more or less arbitrary characters, and that 
although many species are real natural groups of individuals, many others are 
simply arbitrary groups, associated for the sake of convenience. After review- 
ing the various methods of distinguishing species, KUpPFFER* concludes that 
no method will apply in all cases, that all methods are of importance, and that 
when the several methods are used conjointly, little difficulty is experienced. 
KupFFER then turns to the methods of K6LREUTER, based upon the sterility 
of hybrids, as a method which has not been used to the extent its merits warrant. 
Sterility of the hybrids being presumably due to defective germ-cells, he depends 
for his measure of sterility upon the condition of the pollen, basing his method 
upon the fact pointed out a few years ago by JENcIc5 that viable pollen swells 
immediately upon the introduction of water, while the sterile pollen remains 
- Shrunken, and that this capacity of the normal pollen to swell is retained for 
many years in herbarium materials (more than 50 years in Viola, fide KUPFFER). 
Although the author eee that considerable sterility of the pollen has 
been observed in many “good” species, he has himself never found a pure 
species in which more than a few (ein Paar) per cent. of the pollen grains remained 
shrunken, the implication being that the reported instances would bear further 
consideration. 
After examining a number of species and their hybrids, especially among the 
Violaceae, he concludes that when a supposed hybrid shows much less fertility 
of the pollen than its supposed parents, it is not a mecessary but a sufficient proof 
(1) that the supposed hybrid is truly a hybrid, and (2) that its parents belong to 
distinct species, 
Application of this aehed is then made with interesting results to forms 
of Potentilla, Viola, Thymus, etc., which have puzzled the systematist—GEORGE 
H. Suu. 
Propagation of grain rust—Further comments by Dr. JaKosp Ertxsson® 
on the question of the origin and distribution of the rust-diseases of plants have 
recently been presented to the botanical public through separata. The author 
has not essayed so much to put forth new facts, as to bring together and review 
those recently published, in so far as they bear upon his mycoplasm theory, 
giving especial attention to adverse criticisms. 
4Kuprrer, K. R., Kélreuters Methode der Art Abgrenzung nebst Beispielen 
ihrer Anwendung und einigen allgemeinen Betrachtungen iiber legitime and hybride 
Pflanzenformen. Acta Hort. Bot. Univ. Imp. Jurjevensis 6:1-19. 1905. 
5 JENcIc, Untersuchungen des Pollens hybrider Pflanzen. Oesterr. Bot. Zeits. 
$0:1, 41, 81. 1g00. 
6 Errxsson, J. Zur Frage der Sulsictoaing und Verbreitung ie Rostkrank- 
heiten der Pflanzen. Arkiv for Botanik 53:1-54. 1905. 
