34 SPERMATOGENESIS AND FECUNDATION OF ZAMIA. 
a protrusion of the nucellar tissue over it, which makes its course easier 
to follow. It is particularly interesting to note that when several 
tubes develop in the same nucellus they apparently do not grow 
altogether at random, but divide the space almost equally, radiating 
out from the pollen chamber as a center, with almost equal angles 
between them. There is no special structure of the nucellar tissue 
which guides this distribution of the tubes, so far as can be observed, 
and the pollen grains are apparently not guided in their distribution 
or position in the pollen chamber. When a number of grains are 
found in the same nucellus they may be scattered in the lower part of 
the pollen chamber or grouped more or less together. When they 
germinate, and the tubes turn toward and enter the nucellar tissue, two 
or more may be found to enter the tissue and start in the same direc- 
tion. As they elongate, however, they diverge and divide the space 
between them, and this is a very necessary provision, apparently, as 
they are so large that in the thin nucellus of old stages there would 
not be room for them to grow together and cross. The writer has 
examined several thousand ovules in condition to show this feature 
well, and in no instance have two tubes been found very close together. 
In case there are many tubes they are necessarily nearer together than 
when there are only a few tubes. If there are only two or three tubes 
they may not divide the space exactly equally between themselves, 
but are invariably found to be separated by a fairly wide angle. In 
no case have the tubes been found bunched together on one side of 
the nucellus, as would occasionally occur if there were not some force 
guiding their direction of growth. It seems probable that when a 
tube enters the nucellar tissue it may produce a chemical change of 
some sort which serves to repel other tubes. It may, on the other 
hand, be simply a reaction to richness of food supply. Their distri- 
bution and growth in the nucellar tissue seem similar to that of root 
distribution in the soil, exclusive of the factor of light. 
During the growth of the pollen tubes the nucellus has continued to 
change considerably in form and shape. In the early stages of growth 
the apex of the nucellus has a considerable thickness compared with 
the pollen tubes (fig. 5). The tubes at first grow straight out laterally 
in the apical tissue of the nucellus (fig. 5), but before reaching the sur- 
face they turn downward and continue growing down through the 
tissue of the sides of the nucellus just underneath the surface (fig. 51). 
As the development continues the nucellus grows in size, but gradually 
becomes thinner, until at the time of fecundation it is reduced to a thin, 
papery membrane, except at the apex, where other changes have mean- 
while taken place. The tissue below the apex of the nucellus which 
was at first pointed becomes more or less contracted and sunken, as 
shown in-figure 51. The tissue below the pollen chamber and between 
it and the prothallus, which is at first about 2 mm. in thickness, is gradu- 
ally absorbed and finally entirely breaks away, leaving an opening from 
