cotyledonous and Dicotyledonous embryos, which require notice. 1. Monocotyledonous Germination.-The seeds of Monocotyledonous plants, in by far the majority of instances, contain albumen, which, as the embryo develops, is usually entirely absorbed. The single cotyledon of Monocotyledonous seeds, when they contain albumen, always remains entirely (Fig. 111, c) or partially within the integuments during germination. In the latter case, the intra-seminal portion of the cotyledon corresponds to the limb of the cotyledonary leaf, and the portion which elongates beyond the integuments (extraseminal) represents the petiolar portion. The latter part varies much in length, and is commonly terminated by a sheath, which incloses the young axis with the plumule. At other times there is no evident petiolar part, but the sheathing portion enveloping the axis remains sessile on the outside of the seed, and elongates in a tangential direction to it, as in the Oat (Fig. 111), where the cotyledon, c, remains within the seed, and the plumule, g, rises upward from its axil into the air. In some few Monocotyledonous Orders, such as Naiadaceæ, Alismaceæ, etc., where the seeds are exalbuminous, the cotyledon is commonly freed from the integuments, and raised upward with the plumule. As already noticed in the germination of Monocotyledonous embryos, e. g., the Grasses, the radicle is not itself continued downward, so as to form the root, but it gives off one or more branches of nearly equal size, which separately pierce its extremity, and become the rootlets (Fig. III, r). Each of these rootlets, at the point where it pierces the radicular extremity, is surrounded by a cellular sheath termed the root-sheath or coleorhiza, co. This mode of germination is commonly termed endorhizal; but it is by no means universal in the class. 2. Dicotyledonous Germination. The seeds of Dicotyledonous plants are either albuminous or exalbuminous, and their germination in such respects, as a general rule, presents no peculiarity worth notice. The two cotyledons either remain within the integuments of the seed in the form of fleshy lobes, as in the Horse-chestnut and Oak, in which case they are said to be hypogeal; or, as is more commonly the case, they burst through the coats, and rise out of the ground in the form of green leaves, in which case they are epigeal. In the course of development the cotyledons commonly separate, and the plumule comes out from between them. In those cases where they remain within the integuments, they sometimes become more or less united, so that the embryo resembles that of a Monocotyledon; but a Dicotyledonous embryo may be always distinguished from a Monocotyledonous one by its plumule coming out from between the bases of the cotyledons, and not passing through a sheath. The radicle of a Dicotyledonous embryo is itself prolonged downward by cell-multiplication just within its apex (Fig. 109, a), to form the root. An embryo which germinates in this way is termed exorhizal. CHAPTER IV. SPECIAL PHENOMENA IN THE LIFE OF THE PLANT. 1. Development of Heat by Plants.-As the various parts of living plants are the seat of active chemical and other changes during their development, and in the performance of their different functions, we might conclude that their temperature would rarely or ever, under natural circumstances, correspond with that of the atmosphere around them. We have already noticed that, during the germination of seeds, a considerable development of heat takes place (page 263). This is more especially evident when a number of seeds germinate together, as in the process of malting. The development of heat in flowering has also been alluded to (page 241). The rise of temperature which thus occurs in the processes of germination and flowering is due, without doubt, essentially, to the production of carbon dioxide. We have still to inquire whether the ordinary vital actions which are going on in plants are calculated to raise or diminish their temperature. The experiments of several observers, and more especially of Schübeler, lead to the conclusion that the trees of our climate with thick trunks exhibit a variable internal temperature, being higher in the winter and at sunrise than the surrounding atmosphere-that is, at the periods of great cold, or of moderate temperature; and lower in the summer or at mid-day—that is, at periods of great heat. 268 LUMINOSITY OF PLANTS. In no observed cases were such trees noticed to possess exactly the temperature of the atmosphere around them. These and all other conclusions which have been at present arrived at respecting the development of heat by plants leave the question, however, in a very unsatisfactory state, and much further investigation is required upon this mat ter. 2. Luminosity of Plants.-Very little is positively known respecting the development of light by plants. But it seems tolerably well ascertained, on the authority of several observers, that the thallomes of some living Fungi are luminous in the dark. This luminosity or phosphorescence has been noticed in several species of Agaricus and the so-called Rhizomorpha. The mycelium of the common Truffle is also said to be luminous in the dark. With regard to the development of light by the higher classes of plants, we have at present no very satisfactory observations to depend upon. It has been repeatedly stated, that many orange and red-colored flowers, such as those of the Nasturtium, Sunflower, Marigolds, Orange Lilies, Red Poppies, etc., give out, on the evening of a hot day in summer, peculiar flashes of light. This peculiar luminosity of orange and red flowers is now commonly regarded as an optical illusion, and the fact of such luminosity having been only noticed in flowers with such bright and gaudy tints appears strongly to favor such a conclusion. The rhizomes of certain Indian Grasses have been reported to be luminous in the dark during the rainy season; and Mornay and Martius have observed that the milky juices of some plants were luminous when exuding from wounds made in them. Martius also states that the milky juice of Euphorbia phosphorea is luminous after removal from the plant, when it is heated. 3. Electricity of Plants.-Disturbances of electrical equilibrium are undoubtedly connected with the various ELECTRICITY OF PLANTS. 269 chemical and mechanical changes which take place in plants. By the medium of a galvanometer, Ranke, Velten, Burdon Sanderson, and others have demonstrated that there exists in plants an electric current from the transverse to the longitudinal section of a vegetable fiber, similar, but in the contrary direction, to that shown by Du Bois-Reymond to exist in the muscles, etc., of animals. It is also found that the internal tissue of land plants is always electro-negative to the cuticularized surface. The Effect of the Electric Light on the Growth of Plants and Production of Chlorophyl.-Recent experiments made by Dr. Siemens seem to prove that the electric light aids the growth of plants, produces chlorophyl, increases the brilliancy of flowers, and promotes the ripening of fruits. By sowing seeds of rapidly growing plants and exposing them to the same conditions with the exception of light, he found that those grown in the dark were etiolated and soon withered; those exposed to daylight, with a fair share of sunlight, were vigorous, and of a good green color; but those exposed to the electric light for six hours per day only, being in darkness the other eighteen hours, were vigorous though less green; while those exposed to daylight and electric light successively, were the most vigorous, and the green of their leaves of a darker hue. This shows that plants may for a time grow continuously without rest, i. e., without sleep; but, for what length of time this endurance would continue, further experiments are required to prove. The electric light seems therefore to affect plants in a similar manner to the continuous summer sun in northern latitudes, where Dr. Schübeler found that the arctic sun caused plants to produce more brilliant flowers and richer and larger fruit than if the same plants had been grown with an alternation of light and darkness. 4. Movements of Plants.-Three kinds of movements have been described in plants: 1. Motions of entire |