332 
AMERICAN JOURNAL OF BOTANY 
[Vol. io, 
density of the surrounding protoplasm. The three cells remaining at the 
chalazal end of the embryo sac, the antipodal cells, are cut off by cleavage 
planes from the remainder of the embryo sac and stain very deeply. They 
are pressed against the chalaza by the growth of the embryo sac, become 
flattened, and finally disintegrate. 
The fertilization of the egg and the development of the embryo were 
not followed, as these stages were not found in the material at hand. This 
material was collected in early summer from plants of the spring crop, at 
a time when weather conditions are generally unfavorable for the production 
of seed. It is accordingly not strange that stages in fertilization and 
embryo-development were lacking. 
Degenerative Changes in Ovules and Pollen 
The shedding of the potato blossoms and buds is preceded by degenera¬ 
tive changes which may be manifest to the naked eye or visible only upon 
microscopic examination. The macroscopic evidences are such as are 
ordinarily comprehended by the term “blasting.” Growth ceases, the 
buds become yellowish, and wilting ensues. These symptoms are most 
noticeable when the buds are partly grown, and the changes involve both 
the anther contents and the ovules. When the pollen grains degenerate 
at a late stage of development, the ovules may be but slightly involved, 
many of them containing normal embryo sacs with eggs capable of being 
fertilized. In this case, normal blossoming may occur and viable seeds may 
be produced if weather conditions are favorable and if cross-pollination by 
a suitable variety is provided. This is the condition which obtains when 
sterility is due to hereditary causes acting principally upon the pollen. 
In such cases the anther locules are found to be empty, or contain the 
empty, shriveled walls of pollen grains. Under exceptionally favorable 
circumstances a few viable pollen grains may be produced, but there is 
little chance that they will reach the stigma of the blossom and cause 
fertilization, even if the anthers should not remain closed, as often happens 
in such cases. Sterility in this case is due to inherent lack of vitality in 
the pollen, the cells apparently being unable to continue their development 
after the formation of the thick walls cuts the grains off from their source of 
food supply. 
Degenerative changes due to unfavorable conditions of the environment 
may have their inception at any stage in the development of the bud and 
usually involve the contents of the ovary as well as those of the anthers. 
The first indication of change in the archesporial cells is a starved appearance 
(PI. XXVII, fig. 3). Growth and cell division cease. The nutritive cells 
and tapetum stain but lightly and appear to have but very scant contents. 
The archesporial cells become somewhat shrunken, the cytoplasm stains 
lightly, and numerous vacuoles appear, while the nuclei assume an abnormal, 
coarsely granular appearance. A little later the entire sporogenous tissue 
