ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 731 



BOTANY. 



GENERAL, 



Including the Anatomy and Physiology of Seed Plants. 



Structure and Development. 

 Vegetative. 1 



Phyletic Significance of Ray Tracheids in Coniferae.* — W. P. 

 Thompson has published an examination of the origin of the ray tracheids 

 in Coniferae, with a view to supplying a basis for determining their true 

 phylogenetic significance. The author refers, in the latter connexion, 

 to the work of Penhallow, who concludes that the number of ray 

 tracheids in any species is in direct proportion to the degree of its 

 specialization ; and also to the contrary conclusion of Jeffrey, who 

 maintains that the forms with few ray tracheids have been derived by 

 reduction from forms with many. 



The species investigated in detail were types of the hard and soft pines 

 — Pinus resinosa and P. Strobus, the results being checked by compari- 

 son with several others ; the regions specially examined were the primi- 

 tive ones — seedling stem and root, young branch and young root of the 

 adult, and seed-cone axis. In the young root, complete transitions were 

 observed, from short tracheids between the rays to ray tracheids, both 

 marginal and interspersed. The natural conclusion that the latter 

 originate from tracheary tissue is supported by the fact that they 

 develop in the cambial region of the seedling, and by the presence of 

 tail-like projections on the ray tracheids, representing the smooth part 

 of the tracheid as seen at the cambium. The occurrence of these 

 ray tracheids bears a definite relation to that of albuminous cells ; in 

 Abies, the presence of the latter and the traumatic occurrence of ray 

 tracheids indicates their vestigial nature. The regional and fossil 

 distribution of the ray tracheids indicates their ancestral absence in the 

 pines ; hard pines are more specialized than soft ones. The large rays 

 in Pinus are usually formed by the fusion of smaller ones. Ray 

 tracheids are often replaced by parenchyma cells ; the importance of 

 this in the formation of secondary parenchymatous rays is pointed out. 



The author concludes generally that ray tracheids are specialized 

 structures, so that the greater their number the more recent in descent 

 is the form in which they occur. 



Reproductive. 



Morphology and Relationships of the Podocarpinese.f — Mary S. 

 Young has contributed to our knowledge of the morphology of the 



* Bot. Gaz., 1. (1910) pp. 101-16. t Tom. cit., pp. 81-99 (3 pis.). 



3 c 2 



