102 COMPARATIVE EMBRYOLOGY OF THE RUBIACEAE 
Molisch (9). These facts explain its behavior toward the stigma, 
but the later behavior in the conductive tissue remains unexplained. 
According to Molisch again, chemotropism “ muss bei der Wan- 
derung des pollenschlauches zur Eizelle, resp. bei der Auffindung 
derselben in vielen Fallen eine wichtige Rolle zufallen.” But it 
is difficult to see how chemotropism can be an efficient factor 
unless there be a differential distribution of the stimulant. 
Miyoshi on the other hand, has maintained, in the words 
above quoted, that the operation is a purely mechanical one. 
Nawaschin (16), to whom we are indebted for a critical exposition 
of these two views, has pointed out that both presuppose that “ der 
Pollenschlauch Reizbewegungen ungehindert ausführen könne,” 
a condition which is found only where the pollen tubes have free 
space in which to move. If we assume that such are the conditions 
in Diodia Virginiana, in which the pollen tube moves freely in the 
ovarial cavity, what may we say of Diodia teres and Richardsonia 
Pilosa, in which the pollen tube is not free to move in the ovarial 
cavity, but pursues an intercellular path, parallel to that which we 
may believe, from the case of Diodia Virginiana, it would take were 
the mechanical conditions favorable, and this in spite of the char- 
acter of the conductive tissue which is mechanically unfavorable. 
I believe that we may be justified in maintaining : 
t. With Nawaschin, that the occurrence of a secreting con- 
ductive tissue speaks as much for the nutrition of the pollen tube 
as for the necessity of mechanical guidance ; that, moreover, the 
ability of the pollen tube to secrete a cellulose-dissolving enzyme 
indicates the former, as in point of fact the pollen tube does 
not penetrate the cells of the conductive tissue ; and that the as- ۰ 
sumption that in order to respond toa stimulus, the pollen tube 
must have unoccupied space in which to move is unnecessary and 
probably untrue. 
2: With Molisch, that chemotropism is the important factor, 
and the guidance of the pollen tube; but that the distribution of 
the ed in the conductive tissue is a differential one ; and, as 
Nawaschin (16) suggests, “ die Verbreitung der Reizstoffe Kann 
zwischen den Papillen des Leitgewebes in F olge der capillarkrafte 
leicht zu Stande kommen,” extending his statement, however, to 
cells as well as papillae, 
