STERILITIES OF WILD AXD CULTIVATED POTATOES. ( 
media (PI. IV. ties. 1 and 2). Thus, in the best pollen bearers of the cultivated 
varieties there is much impotence of pollen, and the relative number of grains that 
germinate is low. Varieties with such pollen, however, have a record of being able to 
function best as pollen parents. 
Class 2. — -In general appearance and dehiscence anthers of class 2 are like those in 
class 1 : the percentage of plump pollen is low (5 per cent to less than 30 per cent), 
with only a few of these grains germinating. Usually the tubes are of feeble growth, 
but for some varieties listed here tubes 250 n long were observed for a very few grains. 
Green Mountain is fairly typical of this class. The pollen chamber is less spacious 
than in McCormick (PI. I. tig. 6). From the examinations of 30 mounts of pollen, 
mostly on 15 — 1 media it appeared that from 5 to 10 per cent of all grains have granular 
contents but that less than 1 per cent of all pollen could germinate, and the tubes for 
all these were short (PI. V. fig. 1). In some cultures of pollen of the Green Mountain 
no germination was seen. 
The record for crosses with pollen of this class quite uniformly shows poor results. 
Green Mountain selfed onll5 flowers gave only 2 seed balls : on 51 varieties and seedlings 
its pollen failed in 44 combinations, and of the 806 flowers involved only 57 set fruit. 
Yet pollen of Green Mountain on McCormick in one set of crosses gave 27 fruits for 44 
flowers pollinated, an individual record that is exceedingly high for pollen of any 
plant in this class. 
Class i.— Anthers usually well developed, occasionally remaining somewhat 
greenish at apex: dehiscence of nearly all stamens good, but in some varieties poor for 
some stamens: pollen usually scant, but sometimes fairly abundant: percentage of 
plump grains low (usually less than 10 per cent and often less than 1 per cent), rarely 
giving germination, and failing in numerous crosses on varieties known to set fruit 
readily when viable pollen is used. 
As a class, the plants here listed have poorer pollen than those of class 2. Possibly 
the making of a large number of germination tests would show some germination of 
pollen for plants here listed in class 3. The Australian Blue variety is in this class. 
The anthers are large and well developed and dehisce fully, but the pollen chambers 
are narrow (PI. I. tig. 5), pollen is scant, and there is seldom a plump grain. The 
shriveled pollen is aggregated in irregular and rather compact masses, as shown in the 
illustration. The empty and in some cases shrunken pollen grains of the Irish Cobbler 
and Triumph, whirh also belong in this class, are shown in Plate VI, Figures 1 and 2, 
respectively. 
The records of crosses made by the Bureau of Plant Industry show that only a few 
of the varieties of this class have been used as pollen parents in crosses with varieties 
known to be good seed parents. Irish Cobbler pollen occasionally showed a very few 
pollen tubes in the germination tests, but the use of pollen of this variety in crosses on 
12 varieties with a total of 156 flowers failed in every case. Early Rose pollen failed 
completely on 8 varieties involving 83 flowers. Such failures are to be expected from 
the appearance of pollen of these varieties and its lack of germination on media. 
Class 4- — Anthers mostly greenish: dehiscence irregular, with some or even all an- 
thers failing to open; pollen very scant, with scarcely a plump grain, or pollen grains 
lacking. No germination. 
In Rose Xo. 4, which belongs in this class, the anthers are greenish yellow; the de- 
hiscence is irregular, with some anthers failing to open; the pollen is scant, with very 
rarely a plump grain. The Russet Burbank variety has some orange-colored anthers, 
but many are greenish, tapering at apex, indehiscent, and no good pollen was observed . 
Berrick and Sensation illustrate well the condition in which anthers are decidedly 
undersized, greenish, and almost, if not completely, indehiscent. It is obvious that 
varieties belonging in this class an rarely ever function as pollen parents. 
These observations and the tests for germination give results that 
fully support the very general view that pollen sterility is a condition 
which accounts for much of the failure of fruit production in potatoes. 
In the cultivated varieties and also in certain wild species there is a 
very general impotence of pollen. 
The varieties grouped in class 1 are the only ones of those studied 
which can with reason be expected to function at all well as male 
parents, and even in the best of these a large percentage of the pollen 
is impotent. Possibly a few of the plants in class 2 can function oc- 
casionally as pollen parents, and if pollen germinates more readily on 
pistils than on artificial media perhaps varieties in class 3 may some- 
times function as pollen parents under specially favorable conditions. 
