IV 
TABLE OF CONTENTS 
No. 4, April 
Pectinase in the spores of Rhizopus. . . J. L. Weimer and L. L. Harter 167 
The chromosomes of Riccardia pinguis (with Plate XX) 
Amos M. Showalter 170 
Australasian botanical notes II. Victoria, South Australia, and West 
Australia (with Plates XXI and XXII) 
Douglas Houghton Campbell 173 
Evolution and geographical distribution of the genus Vernonia in North 
America (with three text figures). Henry Allan Gleason 187 
The availability of iron in nutrient solutions for wheat (with four text 
figures).W. E. Tottingham and E. J. Rankin 203 
The effect of the hydrogen ion on the protoplasm of the root hairs of 
wheat (with Plate XXIII). Ruth M. Addoms 21 i 
No. 5, May 
The species concept from the point of view of a geneticist 
George H. Shull 221 
The species concept from the point of view of a morphologist 
R. A. Harper 229 
The species concept from the point of view of a physiologist and bac¬ 
teriologist . Guilford Reed 234 
The species concept from the point of view of a plant pathologist 
E. C. Stakman 239 
The relation of the enzym pectinase to infection of sweet potatoes by 
Rhizopus.L. L. Harter and J. L. Weimer 245 
The inheritance of flower types and fertility in the strawberry 
W. D. Valleau 259 
No. 6, June 
Some effects of physiological conditions on genetic characters of wheat 
W. F. Gericke 275 
A quantitative study of anisophylly in Acer (with one text figure) 
Edmund W. Sinnott and George B. Durham 278 
Is it maltose or phloridzin that is abundant in apple tissue? 
E. M. Harvey 288 
Cryptomorpha, a new section of Saxifraga (with two text figures) 
Arthur Monrad Johnson 294 
The Bolivian species of Vernonia. Henry Allan Gleason 297 
Studies of physical and morphological changes in Bartlett pears (with 
three text figures and Plate XXIV). . .Andrew Edward Murneek 310 
The formation and degeneration of germ cells in the potato (with two 
text figures and Plates XXV-XXVII). W. J. Young 325 
The embryo of Lemna (with one text figure and Plate XXVIII) 
Frederick H. Blodgett 336 
