386 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
In spite of the absolutely conclusive work of Camerarius, Koelreuter, 
and Sprengel on the sexuality of plants, their conclusions were often 
rejected during the first half of the nineteenth century. Thus 
certain devotees of the nature philosophy occupied themselves 
either in proving over again, after Cesalpino, that plants can not be 
sexual because of their nature, or in trying, by ill-conceived and 
carelessly performed ‘experiments,’’ to prove the conclusions of 
Camerarius and Koelreuter erroneous. These objectors were finally 
silenced, however, when Gaertner, in 1849, published the results of 
such a large number of well-checked experiments, entirely confirming 
the works of Camerarius, Koelreuter, and Sprengel, that no thinking 
botanist has since doubted the occurrence in flowering plants of a 
sexuality essentially identical with that found in animals. 
IIl.—THE DISCOVERY OF THE POLLEN TUBE AND ITS RELATION TO THE 
ORIGIN OF THE EMBRYO, 1823-1847. 
During the opening years of the nineteenth century a number of 
botanists, who believed in the sexuality of plants, tried to discover 
by the aid of the microscope just how fertilization is effected. Most 
botanists of the day believed the pollen grain burst on the stigma, 
and that its granular contents found a way through the style to the 
ovary. An entirely new aspect of the problem of fertilization was 
opened up, however, when in 1823, Amici, of Modena, saw on the 
stigma of Portulacca young pollen tubes arising from the pollen 
grains. Seven years later he followed these tubes through the style 
to the micropyle of the ovule. At about this time also, Jakob 
Matthias Schleiden (1838) took up the study of this same problem. 
He was a man of vigorous intellect and great versatility, who some- 
times misinterpreted what he saw, but who proved a most stimulating 
opponent to a number of other workers who did observe accurately. 
After denying Robert Brown’s assertion that the pollen tubes of the 
orchids arise in the ovary, Schleiden proceeded to describe and figure 
the pollen tube as penetrating not merely the style and then the 
micropyle, but even far into the embryo sac itself. 
Here, as he says in his Grundziige (I, p. 373): 
The end (of the pollen tube) soon swells, either in such a way that the vesicle arising 
in it fills the whole cavity of the portion of the tube within the embryo sac, or there is 
left, between the apex of the embryo sac and the embryonal vesicle of the tube, a long 
or a short cylindrical portion of the latter, the suspensor. 
He thus regarded the embryo sac as a sort of hatching place for the 
embryo, which he thought formed from the end of the pollen tube. 
This idea of the origin of the embryo really denied the occurrence of 
any actual sexual process, and made the pollen the mother of the 
embryo. 
