8o The Science of Life. 



for more than twenty years he was able to steer clear of 

 the mistakes which misled Brongniart (1826), Robert 

 Brown (1831), and Schleiden (1837), and to prove (1846) 

 that the egg-cell within the embryo-sac of the ovule is 

 stimulated to development by the advent of the end of 

 the pollen-tube. This was at once corroborated by Von 

 Mohl and Hofmeister, and many details have since been 

 added. Strasburger, in particular, has been successful 

 in working out the intricacies of the process, showing 

 that as in animals, so in plants, fertilization is the inti- 

 mate and orderly union of two sex-nuclei, the nucleus of 

 the ovum, and one of the nuclei which arise from the 

 originally single nucleus of the pollen-grain. It would 

 take us beyond our present scope to show how Guignard 

 and others have made the parallelism even closer by 

 comparing the preparatory or maturation processes 

 which precede fertilization in plants and animals alike. 



After the sexuality of Phanerogams had been securely 

 established (1846), attention was turned with fresh 

 Sexuality of confidence to the Cryptogams, in regard to 

 cryptogams, which some important observations had 

 already been made. Thus the antheridia and arche- 

 gonia of mosses had been compared to stamens and 

 ovaries by Schmidel and Hedwig, and the spermato- 

 zoids had been discovered and recognized as such by 

 Unger in 1837. Similar observations had been made 

 by Nageli (1844) and others in regard to the prothallia 

 of ferns. But there was necessarily great obscurity 

 until Hofmeister discovered the alternation of genera- 

 tions (1849), and showed that "the prothallium in 

 the vascular cryptogams is the morphological equiva- 

 lent of the leaf-bearing moss-plant, while the leafy plant 

 of a fern, of a Lycopodium and a rhizocarp answers to 

 the capsule of the moss ". As yet, however, no one 

 had observed the actual union of the male and female 

 sex-elements in Cryptogams, though many botanists 

 had been on the threshold of this discovery. The 

 observation was first made by Pringsheim in the com- 

 mon fresh-water alga CEdogonium, and the fact was 

 immediately confirmed by De Bary. 



Much has since been done (a) in extending the range 



