1915] CURRENT LITERATURE 85 
cent in F;, and 9 per cent in F,. Back crosses with the parents resulted in 
about 6 per cent of cross-sterility. 
Self-sterility was found to be wholly a matter of rate of growth of the 
pollen tubes. The pollen germinated perfectly on stigmas of the same plants, 
but the pollen tubes grew at the rate of about 3 mm. per day, and in no case 
traversed over half the distance to the ovary in the 11 days maximum life oi 
the flowers. Growth of pollen tubes in cross-pollinated styles, on the other 
hand, though starting 4t about the same rate, was so continuously accelerated 
that the ovaries were reached in 4 days or less. 
The simple Mendelian explanations otf self-sterility proposed by CORRENS 
for Cardamine pratensis and by Compton for Reseda odorata’ are not applicable 
to self-sterility i in Nicotiana. East suggests that specific “Indivi idualstoffe”’ 
“call forth the secretion of sugar that gives the direct stimulus” to growth o 
the pollen tube. The hypothesis satisfies the facts presented as regards both 
the total self-sterility in all generations and the slight cross-sterility, which 
increases from generation to generation as the percentage of plants of like 
germinal constitution increases. It occurs to the reviewer that, if the pollen 
or pollen tubes have specific abilities to call forth the growth stimulus in plants 
of unlike germinal constitution, while the stimulus itself (the secreted sugar 
perhaps) is not specific, simultaneous cross and self-pollination of the same 
flower should result in at least partial self-fertility. Evidence derived from 
such pollinations would in any case be oi interest—R. A. EMERSO 
Cytology of the Mucors.—Miss KrEENE* has given an account of the 
development of the zygospores of Sporodinia grandis. She finds at first no 
essential morphological difference in the two sexual branches which give rise 
to the zygospore. Slight differences in size are not regarded as significant. 
Later the branches differ somewhat in their internal structure. The protoplasm 
of one branch is retracted from the cell wall, the intervening space being filled 
with a granular substance. Sometimes there is a slight retraction of the proto- 
plasm of the opposite branch also. The nuclei in the sexual branches are small 
and have the same structure as the nuclei in the rest of the mycelium. They 
appear to increase in number, but divisions were not observed. When the 
sexual branches meet, their walls coalesce in the region of contact. At this 
time a portion of the protoplasm in the end of each branch is delimited either 
. by cleavage furrows which cut in from the walls, or by vacuoles which enlarge 
and cut through the protoplasm to the hyphal wall. In either case a central 
strand remains connecting the protoplasm of the suspensors with that of the 
gametangium. Walls cutting off the gametangium from the suspensors grow in 
*s Bor. Gaz. §7:242-245. 1914 
** KEENE, Miss M. L. Cytological studies of the zygospores of Sporodinia grandis. 
Ann. "wena 28: 455-470. ‘pls. 2. Ig! 
