270 



SCIENCE 



[N. S. Vol. XXIX. No. 737 



variation in the size of the stomatal pores in 

 Tradeseantia ssebrina (with the resulting varia- 

 tion in diffusion capacity) is amply great enough 

 to explain that portion of the daily rise in tran- 

 spiration rate which is not dependent upon the 

 variation in the evaporating power of the air. 

 With direct sunlight it appears that the stomatal 

 variation is not large enough to explain this. 

 The Vegetation of Northern Zacatecas, Meadco: 



Professor P. E. Lloyd, Alabama Polytechnic 



Institute. 



A general comparison between the vegetation 

 of the region indicated in the caption with that 

 of other desert regions, more especially with ihat 

 of the vicinity of Tucson, Ariz., in which the 

 greater general density of the vegetation of the 

 Zacatecas desert is pointed out, and an attempt 

 is made to explain the difference upon the groimds 

 of diversity in meteorological conditions as bear- 

 ing on soil-moisture and evaporation. 



The extended account embraces a year's ob- 

 servations of the meteorology, the topography and 

 soils of an area of 2,000,000 acres, a somewhat 

 detailed account of the plants and their distribu- 

 tion as related to the topography, together with 

 observations upon the seasonal changes, and the 

 adaptational characters in the vegetation which 

 appear to be correlated with them. 

 The Presence and Absence Hypothesis: Dr. G. H. 



Shtjll, Station for Experimental Evolution. 



In explaining the behavior of what are now 

 called Mendelian hybrids, Mendel assumed that 

 pairs of antagonistic characters are represented 

 by pairs of internal units, one member of each 

 such pair of units coming from the one parent, 

 the other from the other parent, and both existing 

 side by side in the heterozygotes. De Vries laid 

 great stress upon this conception in distinguish- 

 ing between " varieties " and " species." This 

 idea is perhaps yet the most commonly held, 

 though it has recently become common to describe 

 a Mendelian " pair " of characters in the terms 

 of the presence and absence of a single character. 

 There is no evidence of the existence of a pair of 

 internal units or " allelomorphs " and the phe- 

 nomena of incomplete dominance, reversal of 

 dominance, etc., can be simply explained by the 

 assumption that there is no paired condition of 

 internal units. The dominance of the absence of 

 a character over its presence is readily explained 

 by analogy with many common chemical reactions, 

 and while it may be assumed, as has been done by 

 Bateson and Davenport, that what appears to be 

 absence of a character may really be the presence 

 of an inhibiting factor, this is not a necessary 



assumption. It can be shown that absence of an 

 internal unit may be expected occasionally to 

 dominate its presence. 



Cultures of Vredinew in 1908: Professor J. C. 



Aethdb, Purdue University. 



For the tenth consecutive season cultures of 

 various species of rusts have been made from both 

 resting or winter spores and active or summer 

 spores. There were 204 collections with resting 

 spores available, of which, however, only 105 col- 

 lections were brought into germinating condition. 

 Of those which could be made to germinate 248 

 sowings were made, representing about 44 differ- 

 ent species. Of collections with active spores 73 

 sowings were made, representing about 16 species. 

 The proportion of successful cultures equaled, or 

 possibly exceeded, that of previous seasons. Some 

 of the results of most general interest may be 

 mentioned. For the first time in America the 

 early or brown rust on rye was grown on Anchusa, 

 being sown in July, and the similar rust on wheat 

 failed to germinate so soon after maturity, seem- 

 ingly settling the much-discussed identity of the 

 so-called Puccinia dispersa of Europe and of 

 America. Many trials with the rust on timothy, 

 P. phleipratensis, failed to infect barberry plants, 

 this agreeing with European studies. Telial con- 

 nections were established for ^oidium macro- 

 sporum on Smilax, and the secia on Ranuncultis, 

 Cymialaria and Aquilegia sp. An unusually large 

 addition to current information was secured re- 

 garding species of Gymnosporangium. The life- 

 cycle was demonstrated for the first time for one 

 species from the southern states, one species from 

 the northern states and one species from the west- 

 ern states beyond the Kocky Mountains. 



Dichotocladium, a New Genus of the Mucorinice: 

 Professor A. F. Blakeslee, Connecticut Agri- 

 cultural College. 



The species (D. stoloniferum) which forma the 

 type of this genus was found five years ago in 

 Venezuela growing saprophytically on dung. It 

 has the habit of growth of a Chcetocladium, to 

 which it is undoubtedly most nearly related. It 

 differs from Ohwtooladium primarily in that the 

 fertile branches are dichotomous, not in whorls, 

 and the sterile ends are not bristle pointed. 



The genus may be briefly characterized as fol- 

 lows: Vegetative hyphse stout, distinct, contin- 

 uous. Fertile hyphse erect or creeping, stolonif- 

 erous, bearing one to several lateral bushy crowns 

 of repeatedly forked hyphae. Ultimate branches 

 of crown, slender, projecting beyond its surface, or 

 short, terminated by persistent swollen heads upon 



