324 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



Hofmeister (1849) observed that the cell nucleus may resolve itself into rod- 

 like bodies (i.e., chromosomes). The conception of the nature and course of 

 fertilization was at that time still fairly vague, but he was able to prove the 

 presence of the egg cell in the ovule before fertilization, and its subsequent 

 development into an embryo. 



Hofmeister, one of the most brilliant investigators in the history of botany, 

 did his most distinguished work in the sphere of comparative morphology 

 (1851). He demonstrated the fundamental agreement in the life history of 

 mosses, vascular cryptogams, and conifers. All are characterized by an alter- 

 nation of generations, one spore-bearing and another exhibiting sexual repro- 

 duction. He explained how seeds are formed: the megaspore is not released 

 from the megasporangium, but germinates there. The pollen grains correspond 

 to the spores of vascular cryptogams. Hofmeister proved that a uniform plan 

 underlies all cormophytes, and that the old opinion of a fundamental difference 

 in the methods of reproduction of vascular cryptogams and phanerogams must 

 be modified. His results supplied the basis for the distinction of five large groups 

 of plants: Thallophyta, Bryophyta, Pteridophyta, Gymnospermae, and Angio- 

 spermae, although it was a long time before this division was accepted by 

 systematists. 



Besides these discoveries of the morphology and ontogenetic development 

 of plants, paleobotanical research also led to some doubt of the truth of the 

 thesis regarding the constancy of species. A real foundation of paleobotany 

 was not laid until in the period from 1820 to about 1850. Brongniart (1849) 

 distinguished in the historical development of the plant world three principal 

 eras, viz., the era of the cryptogams, that of the gymnosperms, and that of the 

 angiosperms. Floras of different characters have thus followed one another in 

 time, and progress has — generally speaking — been from lower to higher forms. 

 He thus helped to lay the foundations of the theory of evolution, although he 

 himself did not draw any such conclusions. The internal structure of fossil 

 plants was also studied to some extent (Goeppert, 1850; Williamson, 1851). 

 linger (1852) presented an account of the development of floras in earlier geo- 

 logical periods. In this he came to the conclusions that the flora of a district 

 is not only affected by external factors, but is also changing internally, that 

 every more recent plant species must have developed from an older, and that 

 there is accordingly an organic connection between them. 



The most important system of the conifers existing by 1850 is that of End- 

 licher ( 1847 ) . Noteworthy features of this are that the Gnetaceae were regarded 

 as an order of the Conif erae, that Ginkgo was deemed to belong to the Taxineae, 

 another order of the conifers. The works of Endlicher and other investigators 

 bear witness to the importance ascribed to paleobotanical research at that time 

 in gymnosperm systematics. 



The Time Up to About 1880 



Darwin's Origi7i of Species (1859), which established the general theory of 

 organic evolution, profoundly changed the point of view from which the prob- 

 lems of taxonomy were regarded. Fundamental agreement in structure was 

 now explained by unity of descent, and the fact of the natural subordination 



