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SCIENCE 



[N. S. Vol. XXXIX. No. 1000 



ber of well-cheeked 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 essen- 

 tially identical with that found in animals. 



n. THE DISCOVERY OF THE POLLEN TUBE AND 



ITS RELATION TO THE ORIGIN OF THE 



EMBRYO, 1823-1847 



During the opening years of the nine- 

 teenth 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 con- 

 tents 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 Sehleiden (1838) took up the 

 study of this same problem. He was a man 

 of vigorous intellect and great versatility, 

 who sometimes misinterpreted what he saw, 

 but who proved a most stimulating oppo- 

 nent 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, 

 Sehleiden 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 (II., 

 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. 



In 1846, however, the error of this con- 

 ception was clearly demonstrated by Amici, 

 who showed that the embryo of the orchids 

 arises from an egg which is already present 

 in the embryo sac when the pollen tube 

 reaches it. It is this pre-existing egg, ac- 

 cording to Amici, that is stimulated to 

 form the embryo by the presence near it 

 of the pollen tube. This view was confi- 

 dently supported by Mohl (1847) and Hof- 

 meister (1847) in the following year, and 

 the controversy with Sehleiden became 

 even more spirited. As Mohl afterward 

 wrote (1863), men were "led astray by 

 their previous conceptions to believe they 

 saw they could not have seen." The dis- 

 pute even approached the acrimonious, as 

 when Sehleiden (1843) says of one 

 worker's figures, "Solehe Praparate sind 

 ohne Zweifel aus den Kopf gezeichnet. " 



Hofmeister, from the beginning of his 

 study of fertilization in seed plants, had 

 sought in the pollen tube for some equiva- 

 lent of the spermatozoids, those motile male 

 cells of the mosses and ferns that had first 

 been understood by Unger in 1837. He 

 was unable, however, to do more than point 

 out the mistake of earlier observers in re- 

 garding the starch grains of the pollen 

 tube as spermatozoids, and to suggest the 

 likelihood that these motile cells might be 

 discovered in the gynmosperms, a predic- 

 tion the fulfilment of which was realized 

 by Ikeno and Webber fifty years later. In 

 his study of pollen tubes Hofmeister 



