74 BOTANICAL GAZETTE [july 



1 



the submerged algal flora of tropical freshwaters the Cyanophyceae also constitute 

 a very important element, though not as preponderant as in the subaerial flora. 

 This freshwater flora is composed of forms in which narrow filaments are much 

 more abundant than broad ones, a fact thought to be related to the small amount 

 of dissolved oxygen in the water. Cladophora and Rhizoclonium are very poorly 

 represented, and Vaucheria and Botrydium are very rare. The Confervales are 

 not very well represented, and probably the Ulotrichales do not attain much 

 development. Spirogj^a, on the other hand, is even more abundant in the tropics 

 than in northern latitudes. The desmids show a marked filamentous tendency, 

 which may be due to poor aeration. Oedogonium is very abundant, and the 

 freshwater red algae are not at all uncommon. — J. M. C. 



Gametophytes and embryo of Libocedrus. — Lawson^^ has added Libocedrus 

 to the investigated Cupressineae and shows that it agrees in all essential characters 

 with the other genera of the group, as may be seen from the following outline of 

 the results. No prothallial cells are formed in the pollen grain, which at shedding 

 contains the generative and tube nuclei. The pollen tube advances very directly 

 to the archegonial chamber, and upon its arrival the body cell divides to form 

 two large and equal male cells. One to three megaspore mother cells occur and 

 each gives rise to a tetrad. The functioning megaspore becomes filled with 

 endosperm tissue in the usual way, and the megaspore membrane is poorly devel- 

 oped. The archegonia range in number from six to twenty-four, and are grouped 

 in a single complex, invested by a single layer of jacket cells. The ventral canal 

 cell, as in other Cupressineae, is represented only by its nucleus. The contents 

 of the pollen tubes are discharged into the common archegonial chamber, so that 

 both male cells in a tube may function. In fertilization the male slips from its 

 cytoplasmic sheath and unites with the egg nucleus. The two nuclei arising 

 from the division of the fusion nucleus pass to the bottom of the egg and by two 

 successive divisions form eight free nuclei before wall-formation. The cells of 

 the proembryo are arranged in the usual three tiers. — ^J. M. C. 



Evolution of the vascular system of ferns. — Tansley^^ has begun the publica- 

 tion of a series of lectures on the evolution of the vascular system of the Fili- 

 cineae, the first considering the origin of the pteridophytes. The lecturer commits 

 himself to the following positions: that bryophytes and pteridophytes have arisen 

 from the algae independently; that the alternation of generations of the former 

 i? ''antithetic," of the latter ''homologous;" that the prevailing habit of dichotomy 

 among the Filicineae indicates ancestors of the dichotomously branching thallus 

 type, in which the "sporangiferous thallus became specialized for assimilatory 

 functions;" that from such forms the "Filicales, Sphenophyllales, and Equise- 



=1 Lawson, Anstruther a., The gametophytes and embryo of the Cupres- 

 sineae with special reference to Libocedrus decurrens. Annals of Botany 21:281-301. 

 pis. 24-26. 1907. 



33 Tansley, a. G., Lectures on the evolution of the filicinean vascular system. I. 

 New Phytol. 6:25-35. 1907, 



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