118 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



and Schiffner [1926]) a sharp distinction was made between fertilization (the 

 union of morphologically different cells) and conjugation (the union of cells 

 of like form). Thwaites (1848), the discoverer of conjugation in diatoms, was 

 the first to regard conjugation as a sexual process without qualification. 



Vaucher (1803) also studied the genus that was later named for him {Vau- 

 cheria) and designated as sex organs the structures now referred to as anthe- 

 ridia and oogonia. Vaucher was far in advance of his time, however, and his 

 conclusions were not accepted. As late as 1847 Nageli, for instance, thought 

 that red algae were tlie only algae that reproduced sexually but even in this 

 assumption he was entirely wrong inasmuch as he believed the tetrasporangia 

 to be the female sex organs. 



The turning point in the outlook of botanists occurred with the appearance 

 of Thuret's papers of 1853 (a and b) and 1854. He established that in Fucus 

 only eggs to which sperms had had access would germinate. Shortly afterward, 

 Pringsheim (1855, 1856) observed the penetration of the sperm into the egg of 

 Vaucheria and Oedogonium. At first the exact function of the sperm in sexual 

 reproduction was not understood, but it seemed doubtful that it actually fused 

 with the egg. Pringsheim's observations were quickly confirmed, however, by 

 a number of workers. Cohn (1855, 1856) established the occurrence of sexuality 

 in Spkaeroplea and Volvox, De Bary (1858a) confirmed the findings of Prings- 

 heim in regard to Vaucheria and Oedogonium, and Pringsheim (1860) pub- 

 lished his observation on Coleochaete. 



The majority of forms studied during this early period showed oogamous 

 sexual reproduction. The first report of a conjugation of motile isogametes was 

 by Pringsheim in 1870 in regard to Pandorina. This genus thus bridged the 

 gap between the oogamous types and the condition as shown by Spirogyra and 

 diatoms. Shortly afterward isogamous sexual reproduction was discovered in 

 a number of other algae and sufficient evidence was brought to bear to dispel 

 the old belief that conjugation and fertilization were different processes. 



Hertwig in 1876, working on a species of sea urchin, showed for the first 

 time that a significant feature of sexual reproduction was the fusion of the 

 gamete nuclei. Schmitz (1879c) observed a similar fusion of nuclei for the 

 first time in plants in Spirogyra and Berthold (1881) next saw it in the brown 

 alga Ectocarpus. In the words of Mobius (1937, p. 345) : 



Die Kryptogamen waren somit mehr phanerogam geworden als die Phanerogamen, 

 t'iir die man zwar die Differenzierung der Geschlecliter und die Notwendigkeit der Bestau- 

 bung erkannt hatte, die aber in Hinsicht auf den eigentliclien Vorgang der Befruchtung 

 noch ganz kryptogam geblieben waren. 



Phylum Chlorophycophyta 



Characterization: This phylum is comprised of a large and diversified assemblage of 

 algae, ranging from motile and nonmotile unicellular forms to massive coenocytic types, 

 such as certain species of Codium. The majority of species are aquatic. Some genera 

 (e.g., Pleurococcus, Trentepohlia. Fritschiella) are terrestrial or subaerial in occurrence. 

 Certain orders (Siphonocladales, Dasycladales) are wholly marine, others (Zygnematales, 

 Oedogoniales) are freshwater in distribution. Several orders and even some genera 

 (e.g., Chlaviydoiiionas. Chulophora) have both marine and freshwater representatives. 



In the majority of species the cell is provided with a definite wall which is usually 

 composed of an inner, and often stratified, cellulosic layer and an outer pectic layer. In 



