DEVELOPMENT OF THE POLLEN TUBE AND PROTHALLUS. BD 
the pollen chamber into the archegonial chamber. The pollen tubes in 
early stages grow straight out laterally, the pollen grains being fre- 
quently above (toward the apex of the tube), so that the tube in ger- 
mination at first pointed downward and then curved to one side in” 
entering the tissue (see two lower tubes in fig. 5). Such tubes as these, 
and also those which develop in other directions, hecome reflexed during 
the changes which occur in the apex of the nucellus, and finally the 
proximal ends of all tubes (the pollen-grain end) are found to be turned 
down so that they point toward the prothallus. A short time before 
fecundation, active growth begins in this end of the tube. The proximal 
ends of the tubes elongate and push down in the pollen chamber farther 
and farther, the cavity becoming larger and changed in shape. Finally 
the tissue of the nucellus below the pollen chamber breaks away and 
the pollen tubes hang down in a cluster in the archegonial chamber. 
They continue to grow in length until, at the time they burst and dis- 
charge the spermatozoids in the process of fecundation, the proximal 
ends. which hang down free from the tissue (fig. 51), have reached a 
length of from 1 to2mm. During this growth and up to the time the 
tube bursts the old pollen grain forms a little protruding end at the tip 
of the tube, which remains covered by the exine of the pollen grain. 
It stains red with safranin in the Flemming triple process, and is thus 
easily differentiated. | 
An extremely interesting fact in connection with the pollen-tube 
development is the migration of the pollen-tube nucleus, which takes 
place apparently at the beginning of the active growth and extension 
of the proximal end of the tube. It will be remembered that when a 
pollen grain germinates the tube nucleus passes out of the pollen 
grain and takes a position near the end of the growing pollen tube, 
and during the entire active period of growth of this end of the tube 
remains relatively in this position, passing farther into the tube as the 
latter extends in length. In the course of the investigations it was 
found that when the active development had begun in the basal end of 
the tube, a nucleus corresponding in size and appearance to the tube 
nucleus was almost invariably found somewhere near the central cell. 
At first it was thought that this was an abnormal occurrence, the pol- 
len-tube nucleus having remained attached to the prothallus in some 
way and failing to migrate into the pollen tube, as it developed, in 
the normal way. Investigations showed that this nucleus, however, 
is normally found near the central cell in this stage of development. 
It would seem, therefore, that either it must be the pollen-tube nucleus 
which has migrated from the apical end of the tube to this location or 
that the pollen tube nucleus divides at some time during the develop- 
ment of the tube, and that one of the two nuclei remains in this loca- 
tion. Juranyi (72) found that in the pollen tubes of J/acrozamia, 
grown in artificial cultures on pieces of soft pear fruit, the pollen 
tube nucleus after a time often divided into two; however, he appar- 
