272 BOTANICAL GAZETTE [march 



pistils are such that each flower type may be pollinated by either of the other 

 2 flower forms. Moreover, the flowers are placed on the axis of the spike so 

 nearly horizontal as to lessen the probability of self-pollination. 



The microspores are ellipsoidal in form and the different sets of stamens 

 show marked differences in size of pollen grains, the higher anthers having the 

 larger pollen, the middle ones intermediate, and the short-stalked stamens the 

 smallest spores. This relation suggests a correspondence with the 3 types of 

 stigmas. Averaging a large number of spores it was found that the mean 

 diameters of the 3 sizes of pollen grains were as 100, 80, and 51, and their vol- 

 umes respectively as 100, 53, and 14. Recalling Halsted's work on Eichhornia 

 crassipes, in which he found that all sizes of poUen grains germinated if given 

 sufficient time, but that the larger spores germinated much more promptly 

 than the smaller, Hazen suggests that prompt germination would be of great 

 advantage in the long-styled Pontederia flowers in which the flowers wither so 

 quickly that a slow germinating spore might not have time to function. 



The author lists observed insect visitors, naming 10 Lepidoptera and 4 

 Hymenoptera, the least skipper, Ancyloxypha numitor Fabr., being the most 

 frequent visitor. Experimental work by the author is in progress on the 

 relative fertility of the dift'erent flowers with various pollen combinations, and 

 its publication is awaited with interest. — -R. B. Wylie. 



Phototropism. — Miss Parr,' working in Hottes' laboratory of the Uni- 

 versity of Illinois, has done an excellent piece of quantitative work on the 

 response of Piloholus to light. The literature on phototropism has been full 

 of conflicting statements and theories, very largely due to the lack of quanti- 

 tative work of the type done by Miss Parr. This work does much to show the 

 reasons for these diverse views and to lay the foundations for substantial 

 progress. The physics department of the University assisted in the control of 

 the delicate instruments used in the measurements of light. It is very desirable 

 at this stage of plant physiology that we get the more general cooperation of 

 well-trained physicists and chemists to aid in transforming plant physiology 

 from a qualitative to a quantitative science. The results of the work can best 

 be presented by quoting the summary: (i) Piloholus responds to the light of 

 all regions of the visible spectrum; (2) the presentation time decreases gradu- 

 ally from red to violet, and there is no indication of intermediate maxima and 

 minima; (3) the presentation time does not vary in direct ratio with the 

 measured value of the energy of the light in the different regions of the spectrum; 

 (4) the presentation time varies in inverse ratio to the square roots of the wave- 

 frequency; (5) the product of the square root of the frequency times the pres- 

 entation time decreases with the decrease in the energy value of the spectral 

 regions and is an approximate constant for a given light source; (6) the spectral 

 energy in its relation to presentation time may be expressed approximately in 



7 Pare, Rosalie, Response of Piloholus to light. Ann. Botany 32:177-205. 

 1918. 



