148 
a well marked constriction in the macrospore is due simply to a 
slightly diminished development of the outer layers of the 
spore-wall in the upper part of the spore, and a trace of this can 
always be found in the older spores. At any rate the two species 
must be regarded as very closely related. 
Explanation of Plate CXLVI. 
The magnification is indicated before each figure. 
Fig. 1. The base of a very young fertile leaf of Pilularia Americana, showing 
the beginning of the sporocarp, m. its apical cell. 
Fig. 2. Longitudinal section of an older sporocarp—F. F. two of t' e young lobes. 
Fig. 3. A similar section of a somewhat older sporocarp. 
Fig. 4. A still older sporocarp, in which the cavities are well developed. 
Fig. 5. Longitudinal section of on older sporocarp, including the peduncle. S. S. 
sori; H. hairs; fb. fibro-vascular bundle of the peduncle. 
Fig. 6. Longitudinal section of about the same age as in Fig. 5, but at right an- 
gles to the peduncle. 
Fig. 7. Longitudinal section of the young sorus, a. b. c. young sporangia. 
Fig. 8. Transverse section of an older sporocarp, showing the four cavities. 
Fig. 9. A single cavity from a somewhat younger one. 
Figs. 10, 11, Young sporangia, t. the tapetum. 
Fig. 12. Part of the wall of a nearly full grown sporocarp, a. b. c. the outer 
thick-walled cells. 
Free Nitrogen Assimilation by Plants. 
By H. W. Conn. 
The study of bacteriology has introduced to us an entirely new 
realm of knowledge. Twenty-five years ago the scientific world 
had little conception of the great change that was to be made in 
our knowledge of the processes of nature by the development of 
the study of micro-organisms. That bacteria were the cause of cer- 
tain diseases was even then strongly believed by many and had 
been definitely proved in a few cases. But that micro-organisms, 
in general, lay at the foundation of many of the most important 
physiological processes of nature was not even dreamed of. The dif- 
ficulty of research in this line made it possible for only a very 
