gS Morphology BOOK i 



importance. It is impossible to give an account of the 

 work of the numerous observers who helped to elucidate 

 the structure and development of the ovule, and to show 

 its various modifications. Place must be found, however, 

 for Warming's great memoir of 1877, an( ^ ^ or the work of 

 Strasburger in 1877 and 1879, prior to which the embryo- 

 sac was thought to be produced by the simple enlargement 

 of a cell of the nucellus of the ovule, as stated by Sachs. 



The elucidation of the exact nature of the homologies 

 of the pollen tube and its contents was gradually accom- 

 plished, but was not complete till near the end of the 

 century. Soon after the opening of our period Millardet 

 and Pfeffer showed that they could recognize indications 

 of a male prothallium and an antheridium in a very simple 

 condition in the pollen grain of Gymnosperms, when 

 comparing the latter with the microspores of Selaginella 

 and Isoetes, but the nature of the agent of fertilization 

 was not known to them. Sachs spoke of it in the early 

 editions of the Lehrbuch as ' the male fertilizing principle 

 which, passing into the oosphere or embryonic vesicle, 

 causes it to develop the embryo '. He considered the 

 pollen grains to be male sexual cells, and was either not 

 aware of the nuclear divisions taking place in them after 

 maturity, or he attached no importance to them. 



The two nuclei which mark the commencement of ger- 

 mination in the pollen of the Angiosperms were first 

 observed by Hartig in 1866, but their importance was not 

 realized. 



The first real advances of knowledge in this direction 

 appeared in Strasburger's Befruchtung und Zelltheilung, 

 published in 1877, where the tube and its nuclei were 

 discussed. Strasburger showed that the microspores of 

 the Angiosperms can be compared in detail with those of 

 the Gymnosperms so far as their contents are concerned. 

 In this work he erroneously attributed to the vegetative 



