Trelease.] 
410 
[March 15, 
General Meeting. March 1, 1882. 
The President, Mr. S. H. Scudder, in the chair. Twenty 
persons present. 
Dr. W. S. Bigelow gave a historic sketch of the theory of 
spontaneous generation. 
General Meeting. March 15, 1882. 
The President, Mr. S. H. Scudder, in the chair. Twenty- 
persons present. 
The following paper was read : 
ON THE STRUCTURES WHICH FAVOR CROSS-FERTILIZA- 
TION IN SEVERAL PLANTS. 
BY WILLIAM TRELEASE. 
The following paper contains recent or unpublished observa- 
tions on a considerable number of species, chiefly exotic. For 
the most part they were studied at the Botanic Garden in Cam- 
bridge, and I am under great obligation to Dr. Gray and Mr. 
W atson both for the material used and for assistance in determin- 
ing several species. 
Lemnaceae. 
Lemna minor (United States).— In its ordinary vegetative 
condition the plant consists of a flattened ovoid frond 2-4 mm. in 
diameter, bearing a single root. Internal differentiation goes 
scarcely further. An epidermis, bearing stomata on the upper 
side, scattered pigment and raphis cells, the parenchyma so 
arranged as to include numerous large air-cavities, and a poorly- 
developed fibro-vascular structure are the only indications of tis- 
