rEBKUAEY 27, 1914] 



SCIENCE 



303 



demonstrated to his own satisfaction that 

 the tube does not open in accomplishing 

 fertilization. His view, which was the one 

 current till 1884, was that the egg is stimu- 

 lated to develop into the embryo by some 

 substance that diffuses through the imper- 

 forate wall of the pollen tube. 



III. THE DISCOVERY OF A PROTOPLASMIC 

 FUSION AT FERTILIZATION 



We come now to consider a series of dis- 

 coveries of supreme importance in the in- 

 vestigation of the essential sexual process 

 in plants. This is the period in which the 

 problem that had baffled naturalists for 

 twenty centuries was at last solved, at least 

 in one most essential feature, by the demon- 

 stration of the occurrence at fertilization 

 of a mingling of paternal and maternal 

 substances. 



It will not be without interest at this 

 point to note the intellectual stimuli which 

 led an unusual number of workers to in- 

 vestigate this phase of our problem. 



In the first place there were on record, 

 and under discussion at the middle of last 

 century the many puzzling observations 

 of the "Spiral Faden," or animalculse, as 

 they were thought to be, that had been 

 found arising from a number of plants. 

 These motile, spiral filaments had been 

 seen in a liverwort (Fossombromia) by 

 Schmiedel (1747), in Sphagnum by Esen- 

 beck (1822), in Chara by Bischoff (1828), 

 and finally, on the fern prothallus by 

 NaegeU (1844). Unger (1834-37) studied 

 these bodies in the mosses {Sphagnum 

 and Marchantia) and declared his belief 

 that they are not infusoria, but are the 

 male fertilizing cells. At this time also the 

 zoologists of the day were making the first 

 detailed studies of the spermatozoa of ani- 

 mals. Barry (1844) had seen a sperma- 

 tozoon within the egg of the rabbit; 

 Leuckart (1849) saw them enter the frogs' 



egg and then, in 1851, Bischoff and Allen 

 Thompson proved that fertilization is ac- 

 complished by the actual entrance of the 

 spermatozoon into the egg. A no less im- 

 portant influence, in stimulating the botan- 

 ical workers on the problem of fertilization, 

 was the magnificent work of Hofmeister, 

 on the reproductive structures of the 

 mosses, ferns and conifers. By these splen- 

 did researches he had indicated to men of 

 less insight, and less comprehensive imagi- 

 nation, just the points in the life cycles of 

 plants where the critical phases of the re- 

 productive process are to be sought. 



Among the many workers engaged on 

 this problem of fertilization in plants in 

 the third quarter of last century there 

 was, in consequence of readier exchange 

 of information, an attitude of greater con- 

 sideration for the work of other investi- 

 gators than was found in the two pre- 

 ceding decades. There were differences of 

 opinion and interpretation, to be sure, but 

 there was less of that strenuous cocksure- 

 ness when men saw, or thought they saw, 

 differently from others. The mistakes of 

 the brilliant Schleiden were perhaps re- 

 membered. Men like Hofmeister, Prings- 

 heim and Strasburger added to and modi- 

 fied the interpretations of other workers in 

 the same spirit with which they remolded 

 their own immature conclusions. There 

 was a spirit of cooperation evident; it be- 

 came possible for a worker to observe and 

 record the fate of a pollen tube in good 

 temper and with calm judgment. 



The first steps toward the demonstration 

 of a union of two masses of living substance 

 at fertilization resulted from the study of 

 a group of plants, the algae, in which sexu- 

 ality had not been proven or generally ad- 

 mitted. It had, however, long before, been 

 suggested in the case of Spirogyra by Hed- 

 wig (1798) and Vaucher (1803). 



The algffi were in fact especially advan-- 



