1917] SAWYER—IRIS 161 
is continued beyond the stigma. By removing the covers of the 
groove the canal containing the tubes lies exposed, and they can be 
removed with needles. 
Tubes grown in hanging drops of culture media show a rather 
striking tendency to grow from the margins of the drop in groups, 
the tubes being in close contact with each other. This fact is sug- 
gestive when associated with the fact that from the stigma the 
pollen tubes converge in two lines and traverse the stylar branch at 
the margins of the groove, where the space is most restricted 
(fig. 2). It has usually been assumed that the pollen tube is guided 
in its direction of growth by chemotaxis. These observations 
suggest that in Jris contact stimulus may be an effective guide. 
Further investigation of this point is purposed. _ 
The structure of the newly formed pollen grain (fig. 3) was 
studied in sections of the anther. The division of the microspore 
nucleus was not observed, but it occurs in the anther, as would be 
expected, and the generative cell is organized before dehiscence _ 
(fig. 4). The generative cell is slender, elongated, and somewhat 2 
pointed when seen from the side (figs. 6, 13), and is fi over- 
laid by the tube nucleus (figs. a: 5). In this condition the pollen — 
is tin _ Hand pollination was successfully performed. ‘Usually 
d, bu 
speed fies oad a ees the si 
t it was demonstrated ne eee ae 
under which i it was located. “Observation on the rate of growth - SS 
ny Be oes e, and thei sa ieths se 
measured with an ocular micrometer. T 2 ndicates 
