2 Biologie. — Morphologie etc. 



By artificially depressing the branches by means of weights, it 

 was found that strains in the wood cause the branch to become tem- 

 porarily depressed by a small amount. 



The degree of moisture in the atmosphere, as shown by rela- 

 tive humidity observations, has apparenlly a negligible effect on the 

 Position of the branch. 



Modulus of elasticity experiments seem to show that there is a 

 stiffening in the branch below freezing. 



Observations on frost cracks of the linden have shown that the 

 width of the longitudinal cracks follow the changes in air tempera- 

 ture approximately below 32°. When the crack first opens it is not 

 thermometric because it is greatly affected by strain condition, and 

 resistance in the tearing of the wood. 



Thermometers placed withiri the tree trunk at different distan- 

 ces from the surface indicated that the cracks are due to excessive 

 tension conditions in the living portion of the tree directly below 

 the surface of the trunk. 



All observaticns made indicate in a general way that the ben- 

 ding of the branches is closely related to the process which produced 

 the longitudinal frost cracks in the linden. 



Preliminary measurements indicate that the coefficient of expan- 

 sion in the living linden in the transverse directions of the wood 

 directl}'^ below the bark is very large below freezing, while small 

 in the longitudinal direction. 



Experiments on Paulownia tomentosus and Platanus orientalis 

 show scarcely a trace of the thermometric movement as exhibited 

 by the linden. Jongmans. 



Merriman, M. L., Nuclear division of Spirogyra. IL Nuclear 

 di Vision in S. bellis. (Bot. Gazette. LXI. p. 311 — 324. PI. 18—20. 

 1916.) 



Instead of a spireme, as in S. cyassa, a disk arises from material 

 condensing within the mass of nuclear plasm and central body. No 

 trace of Organisation is to be seen in the living disk. Fixed material 

 shows it to arise from aggregations of variable appearance and stai- 

 ning qualities. The more deeply stained arise from the nuclear 

 plasm, the less deeply stained appear to come from thedecompo.sing 

 central body. 



This sphere of aggregated material gradually changes in shape. 

 becoming a cylinder. The more deeply stained masses become 

 arranged upon it as an equatorial band. This band is homologous 

 with the disk seen in living material. As the disk evolves, chroma- 

 tic bodies, averaging 14 for this species, are to be seen on the 

 band, while other irregulär masses of chromatic material project as 

 loops or pyramidal masses from its edge. These loops or masses 

 represent material from nuclear plasm and central body that has 

 partially amalgamated. 



No rift appears in the living disk to indicate a sharp Splitting 

 of components, but instead the changes in appearance indicate a 

 thinning in the center, while parts reassemble at either pole. The 

 chromatic bodies in the fixed disks appear as viscous masses that, 

 as they amalgamate, elongate, while other disconnected chromatic 

 masses are discharged into the cytoplasm as the disk separates 

 into the halves passing to the poles. 



The living disks may be seen sometimes to pass en masse to 



