234 



SCIENCE 



[N. S. Vol. XXIX. No. 736 



plant. Incidentally it becomes a handy refer- 

 ence to the literature of the plant diseases in- 

 vestigated at this station. 



H. S. Jackson's address on the "De- 

 velopment of Disease-Resistant Varieties of 

 Plants," given before the Massachusetts Horti- 

 cultural Society, March 14, 1908, and printed 

 in its proceedings, is a clear popular discussion 

 of a most important but poorly understood 

 subject. Eecognizing that "spraying is a 

 nuisance at best " the speaker urged that more 

 attention should be given to the development 

 of varieties which are resistant to disease, sug- 

 gesting (a) the selection of individuals, (h) 

 the selection of varieties and (c) hybridiza- 

 tion followed by selection of varieties and in- 

 dividuals. Brief discussions of what has been 

 accomplished in regard to wheat, clover, cow 

 peas, potatoes, tobacco, cotton, etc., are given 

 which will astonish those who have not fol- 

 lowed the work of the last few years ; the diffi- 

 culties are candidly pointed out, and some- 

 thing is said as to the cause of immunity, and 

 the possibility of artificial immunity. 



Dr. E. M. East's paper, entitled " A Study of 

 the Factors Influencing the Improvement of 

 the Potato" (Bull. 12Y, 111. Expt. Station) 

 is valuable not only from a practical stand- 

 point, but also for the history of the potato 

 which is given in the introductory pages. 

 After this, methods of breeding, inheritance 

 of characters in tuber selections (in which the 

 author regards the gain as doubtful), degen- 

 eration of varieties (the author concluding 

 that varieties do not run out), mutations, etc., 

 are taken up in succession. The author sug- 

 gests three possible methods of improvement, 

 viz., (1) crossing of desirable plants, (2) selec- 

 tions of the most desirable fluctuations among 

 the plants and tubers of a variety, (3) selec- 

 tion of discontinuous variations and a study of 

 ways of causing them, and regards the first as 

 most promising. 



Here should be noticed J. E. Eockwell's 

 " Index to Papers relating to Plant-Industry 

 Subjects in the Tear-books of the United 

 States Department of Agriculture," which will 

 save much time and labor to the botanist who 

 has occasion to refer to the many valuable 



botanical papers published in the agricultural 

 year-books. 



Three recent papers by Dr. Kraemer in the 

 American Journal of Pharmacy are of in- 

 terest to the plant histologist as well as the 

 pharmacist, viz., " Microscopical and Chemical 

 Examinations of Black Pepper," the same for 

 commercial ginger, and some distinguishing 

 characters of belladonna and scopolia. Each 

 paper is well illustrated by many clear figures. 

 Charles E. Besset 



The UNrvEKSiTT of Nebraska. 



SPECIAL ARTICLES 



SEX DETERMINATION AND PARTHENOGENESIS IN 

 PHYLLOXEEANS AND APHIDS 



The phylloxera of the hickories offer ex- 

 ceptional opportunities for a study of sex- 

 determination and parthenogenesis. In some 

 species three generations can be followed 

 within the same gall — ^two parthenogenetic 

 and one sexual. We can determine the num- 

 ber of males and females that have descended 

 from the same fertilized egg, as well as the 

 influence of external conditions in affecting 

 the number and kind of individuals in each 

 generation. Immense numbers of eggs can 

 be obtained. They furnish also excellent, 

 although difficult, cytological material. 



During three years I have studied the 

 cytological aspects of the life cycle and can 

 now present an almost complete account of 

 the remarkable chromosomal changes that oc- 

 cur in connection with sex-determination. 



I wish to lay especial emphasis on three 

 points : 



1. In many insects it has been found that 

 sex is connected with, or produced by, two 

 kinds of spermatozoa. But in phylloxerans, 

 aphids, bees, ants, in certain saw-flies, in 

 daphnians and in hydatina, the fertilized eggs 

 produce only females. In the phylloxerans 

 and aphids the result is connected with the 

 formation of only functional female-producing 

 spermatozoa — ^the male-producing sperms de- 

 generate. One may suspect that similar con- 

 ditions are to be found in the other groups, 

 and the facts of spermatogenesis in the bee, 

 wasp and ant, support such a view. 



