TWENTY- THIBD ANNUAL MEETING. 81 



other plants of various families. One notable plant of this character is a certain 

 variety of Hosackia Purshiana, whose leaves turn eastward in the morning and west- 

 ward in the afternoon. To facilitate this, all the branches and leaves grow toward 

 the north and south, so that the leaves simply have to turn over. In the case of the 

 Silphium laciniatum, where the two surfaces of the leaf are just about alike, the 

 leaves do not turn one face upward, but stand vertical, with one face to the east 

 and the other to the west, and with the edges north and south. Thus, the leaves, 

 which are rigid, get the benefit of all the sunlight; and the plant only grows on an 

 open prairie, in a sunshiny country. 



AN IMPROVED FIRE-SCREEN; AND NOTES ON THE THERMAL RE- 

 SISTANCE OF FIRE-SCREENS. 



BY T. H. DINSMOEE, EMPOBIA. 

 [Abstract.] 

 This fire-guard is so constructed as to secure neatness of design and unusual se- 

 curity from fire. The subject of thermal resistance of fire-screens is reviewed, and 

 it is shown that resistance varies directly with the distance from the fire, and in- 

 versely with the meshes of the guard. 



METHODS OF COLLECTING, CLEANING AND MOUNTING DIATOMS. 



BY GEETKUDE CEOTTY, LAWEENOE. 



The diatomaceae, called by the writers of the early part of this century bacil- 

 lariacete, are now generally considered to be unicellular microscopical algese. This 

 classification is based upon their usual method of reproduction — that by fission or 

 division. Several eminent scientists, however, as Dr. Farlow, of Harvard College, 

 yet hold that, on account of their freedom of motion, they ought to be placed in the 

 animal kingdom. 



The first forms of this microscopical plant were discovered at the close of the 

 last century by the celebrated Danish micrographer, O. F. Miiller. There have now 

 been described over ten thousand varieties. This number is enormous, considering 

 the fact that American forms have been little studied. With the exception of the 

 late Prof. H. L. Smith, America has not produced what Dr. Henri Van Heurck means 

 by his term "diatomophile." Owing to the fact that allowance has not been made 

 for degeneration in size of any variety, due to division, it is very probable that this 

 number has been unnecessarily increased. 



Each individual diatom is incased within a silicious covering. This covering is 

 composed of two halves, or valves, one fitting over the other as does a lid over a 

 box. The point of union, the point where these two valves overlap one another, is 

 termed the connective zone or girdle. These valves are marked by rows of punct- 

 ures, or stria, except at the center, and frequently each end. The unstriated 

 points are the nodules. These nodules are united by a line, the raphe, or 

 median line. Each cell or plant contains a nucleus and nucliolus, protoplasm 

 and a greenish-brown substance — the endochrome; likewise, several oily globules. 

 Each plant is also covered by a mucilaginous coating, which may be detected by 



