1909] CURRENT LITERATURE 



485 



in which free nuclear division occurs 



rrn 



permanent 



tissue. The archegonia are usually four in number, and a distinct ventral canal 

 cell is cut off, the membrane persisting until fertilization occurs. The proembryo 

 is the usual one of Abietineae, walls appearing at the eight-nucleate stage, and 

 four tiers of cells being organized, the uppermost tier being open.— J. M. C. 



Embryo sac of Pandanus.— Campbells 2 has published the details of the 

 development of the embryo sac of this interesting form, the preliminary account 

 having been published last year.^3 The stage showing fertilization was not 

 obtained, so that it is not certain that the fourteen-nucleate condition described 

 is the fertilization stage. The megaspore mother cell is overlaid by several 

 layers of parietal cells, which are thought to be derived from a single cell. The 

 division of the mother cell is followed by the direct production of the embryo 

 sac by the inner daughter cell. At the first division, the two nuclei assume the 

 polar positions, and subsequent divisions result in two micropylar nuclei and 

 twelve antipodal nuclei. If two megaspore nuclei are supposed to enter into the 

 structure of this sac, there is a single division of one of them, and a succession of 

 divisions from the other one. From any point of view, such a sac would be 

 unusual, and the author is inclined to regard it as "a new type with its nearest 

 analogue in Peperomia," a type which is probably more ancient than the prevail- 

 ing eight-nucleate sac. He dissents from the idea that the reduction division 

 necessarily determines a megaspore in angiosperms, believing that this event 

 may so shift in the life-history that a megaspore may be defined regardless of it. 

 After all, this is merely a matter of definition, and that is a matter of agreement. 

 Shall a megaspore be defined as the product of the two reduction divisions or 

 as the cell which produces the embryo sac? Which definition will have the 

 greater morphological stability? — J. M. C. 



Diffusion of C0 2 in leaves.— That C0 2 does not diffuse extensively through 

 the mesophyll has been known for more than thirty years from the researches 

 of Moll, and experiments to show the localization of photosynthesis have become 

 common in every physiological laboratory. Under Moll's direction, Zijlstra 

 has investigated the extent of diffusion in leaves of different structures. 34 He 

 finds that in all leaves the CO a formed in the leaf itself suffices to produce a line 

 or zone of starch at the edge of the darkened region. In net-veined leaves the 

 movement of CO a supplied from the air is prevented by the larger veins which 



32 Campbell, D. H., The embryo sac of Pandanus. Bull. Torr. Bot. Club 

 36:205-220. pis. 16, 17. 1909. 



33 Campbell, D. H., The embryo sac of Pandanus. Preliminary note. Annals 



of Botany 22:330. 1908. 



34 Zijlstra, K., Kohlensauretransport in Bliittern. Proefschrift ter verkrijging 

 aan der graad van Doctor in plant- en dierkunde aan der Rijks-Universiteit te Gro- 

 ningen 8vo. pp. 128. pis. 2. figs. 2. Groningen: M. de Waal. 1909. 



