260 PART III. VEGETABLE PHYSIOLOGY. 



spring is to be sought for in the double parentage. The offspring 

 of two individuals, or the product of two distinct lines of descent, 

 must occasionally, at least, possess stronger characters — charac- 

 ters better adapted to insure the survival of the individual — than 

 would be possible where the selection is made from one individual, 

 or from one line of descent. Hence, the adoption and continu- 

 ance of the more costly process of sexual reproduction is not an 

 instance of extravagance on the part of nature, but is rather 

 a wise economy of her forces — an investment which brings a 

 profitable return. 



Practical Exercises. 



i. The stems, branches and leaves of various species of Chara and Nitella, 

 the transparent leaf-cells of Vallisneria spiralis, the young stinging-hairs of the 

 Nettle, the hairs on the filaments of Tradescantia Virginica, those on the fila- 

 ments of Linaria vulgaris and those on the filaments of Lobelia syphilitica 

 afford favorable objects for the study of protoplasmic movements in cells. The 

 species of Characese which have the most transparent stems, are the ones to be 

 preferred, and only low magnifying powers, say twenty-five or fifty diameters, 

 are required for their study. The cells of Vallisnefia require a higher magnify- 

 ing power, preferably one of about two hundred diameters. The stamen and 

 Nettle hairs require still higher powers, four hundred or five hundred diame- 

 ters. When stamen-hairs are used, it is better usually to obtain those from 

 unopened flowers. The cells are then more transparent and the movements 

 may be more easily observed. All these objects should be mounted in water 

 with as little crushing or injury to the tissues as possible, as rough handling will 

 often stop the movements of the protoplasm. 



2. For the study of the change of position which chlorophyll-bodies under- 

 go under the influence of light, the common Duckweed (Lemna trisulca), the 

 leaves of such Mosses as Bryum roseum and Funaria hygrometrica or the pro- 

 thallia of any ordinary Fern, being thin and transparent and having large chlo- 

 rophyll-bodies, are favorable objects for study. In experimenting upon them, 

 one part of the organ may be exposed to diffused light by covering it with paper, 

 while the rest is exposed to direct sunlight. After sufficient exposure, the 

 organ may be immediately examined microscopically, or the protoplasm may be 

 first fixed by immersing the organ in alcohol or in solution of iodine. If the 

 chlorophyll-bodies are rendered too transparent by the bleaching action of the 

 alcohol, they may readily be stained by immersing them for a time in very 

 dilute solution of methyl violet. The parts exposed to different conditions of 

 illuminations may then be directly compared. 



3. Take a few seeds of the common Bean, Vicia faba, and cause them to 

 germinate on a damp sponge under a bell-jar. After the radicle has protruded 

 from the seed-coats about one-sixth of an inch, remove one or two of the seeds 

 from the surface of the sponge and fasten them by means of a little shellac 

 cement to a small block of wood, in such a manner that the radicles will project 



