96 COMPARATIVE EMBRYOLOGY OF THE RUBIACEAE 
said in passing that recently Murbeck (10) has, we believe, suc- 
cessfully, contested the‘view of Nawaschin. I believe that a study 
of the facts in a larger number of plants will lead to a more gen- 
eral recognition of the principle laid down by Murbeck (10) in the 
words, “ Der intercellulare Wachstumsmodus des Pollenschlauches 
istal itzubezeichnen, * * e 
and of the mechanical conditions as constituting not the least im- 
portant factors in leading to such behavior. As a further illustra- 
tion of peculiar mechanical conditions 1 would cite the instance 
of Cynomorium in which, as shown by Pirotta and Longo (18), the 
micropyle is completely closed and there appears in its place a cone 
of conductive tissue through which the pollen tube must pene- 
trate. Zinger (21) has shown somewhat similar relations in Can- 
nabis. 
When, as shown above, the pollen tube passes to the surface, 
it travels a short distance along the conductive tissue, soon finding 
its way between the strophiole and the integument. Under such 
circumstances the wall of the pollen tube may well be seen ( fig. 6): 
The effect of the pollen tubes upon the cells of the collar of 
conductive tissue appears to be purely mechanical, as in the case 
of that in the style. Forcing its way between the cells, the pollen 
tube causes in them but little or no distortion and, so far as I have 
observed, no destruction (figs. 5, 6). It would appear that, 
although the cells of the conductive tissue form a true tissue, and 
are not crowded papillae, their thickened walls are nevertheless 
not bound together firmly but rather are loosely related by a soft, 
mucilaginous middle lamella. 
Having reached the micropyle, the pollen tube now passes 
into the micropylar canal. The cells lining the latter are char- 
acterized by somewhat thicker, more deeply staining walls than 
those of the other integumental cells. The pollen tube eventually 
reaches the synergids. In the plants under discussion these cells 
have the “ hyaline, striated tips," as described by Strasburger (21), 
extremely well developed. The generally accepted view that the 
synergids serve as a conductive apparatus is strengthened by the 
analogy which 1 venture to believe exists between their curious 
cellulose tips and the thickened and partially gelatinized cell walls 
of the conductive tissue. 
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