Nerve-physiology has made immensely important advances. There is now good evidence that all excitation of one group of nerve-centres is accompanied by the concurrent inhibition of a whole series of groups of other centres, whose activity might interfere with that of the group excited to action. In a simple reflex flexure of the knee the motor-neurones to the flexor muscles are excited, but concurrently the motor-neurones to the extensor muscles are thrown into a state of inhibition, and so equally with all the varied excitations of the nervous system controlling the movements and activities of the entire body.
Fig. 34.
Diagrammatic representation of the structures present in a typical cell (after Wilson). Note the two centrosomes, sometimes single.
The discovery of the continuity of the protoplasm through the walls of the vegetable cells by means of connecting canals and threads (see [fig. 33]) is one of the most startling facts discovered in connection with plant-structure, since it was held twenty years ago that a fundamental distinction between animal and vegetable structure consisted in the boxing-up or encasement of each vegetable cell-unit in a case of cellulose, whereas animal cells were not so imprisoned, but freely communicated with one another. It perhaps is on this account the less surprising that lately something like sense-organs have been discovered on the roots, stems, and leaves of plants, which, like the otocysts of some animals, appear to be really ‘statocytes,’ and to exert a varying pressure according to the relations of these parts of the plant to gravity. There is apparently something resembling a perception of the incidence of gravity in plants which reacts on irritable tissues, and is the explanation of the phenomena of geotropism. These results have grown out of the observations of Charles Darwin, followed by those of F. Darwin, Haberlandt, and Nemec.
Fig. 35.—The Number of the Chromosomes: (a) Cell of the asexual generation of the cryptogam Pellia epiphylla: the nucleus is about to divide, a polar ray-formation is present at each end of the spindle-shaped nucleus, the chromosomes have divided into two horizontal groups each of sixteen pieces: sixteen is the number of the chromosomes of the ordinary tissue cells of Pellia. (b) Cell of the sexual generation of the same plant (Pellia) in the same phase of division, but with the reduced number of chromosomes—namely, eight in each half of the dividing nucleus. The completed cells of the sexual generation have only eight chromosomes. (c) Somatic or tissue cell of Salamander showing twenty-four ∨-shaped chromosomes, each of which is becoming longitudinally split as a preliminary to division. (d) Sperm-mother-cell from testis of Salamander, showing the reduced number of chromosomes of the sexual cells—namely, twelve; each is split longitudinally. (From original drawings by Prof. Farmer and Mr. Moore.)