Fig. 32.
Bacillus radicola, the parasite which infests the roots of leguminous plants and causes the growth of nodules whilst assisting the plant in the assimilation of nitrogen: (a) Nodule of the roots of the common Lupine, natural size; (b) longitudinal section through a Lupine root and nodule; (c) a single cell from a Lupine nodule showing the bacteria or bacilli, as black particles in the protoplasm, magnified 600 diameters; (d) bacilli from the root nodule of the Lupine; (e) triangular forms of the bacillus from the root nodules of the Vetch; (f) oval forms from the root nodules of the Lupine; (d e f) are magnified 1,500 diameters.
Physiology of Plants and Animals.—Since I have not space to do more than pick out the most important advances in each subject for brief mention, I must signalize in regard to the physiology of plants the better understanding of the function of leaf-green or chlorophyll due to Pringsheim and to the Russian Timiriaseff, the new facts as to the activity of stomata in transpiration discovered by Horace Brown, and the fixation of free nitrogen by living organisms in the soil and by organisms (Bacillus radicola) parasitic in the rootlets of leguminous plants (see [fig. 32]), which thus benefit by a supply of nitrogenous compounds which they can assimilate.
Great progress in the knowledge of the chemistry of the living cells or protoplasm of both plants and animals has been made by the discovery of the fact that ferments or enzymes are not only secreted externally by cells, but exist active and preformed inside cells. Büchner’s final conquest of the secret of the yeast-cell by heroic mechanical methods—the actual grinding to powder of these already very minute bodies—first established this, and now successive discoveries of intracellular ferments have led to the conclusion that it is probable that the cell respires by means of a respiratory ‘oxydase,’ builds up new compounds and destroys existing ones, contracts and accomplishes its own internal life by ferments. Life thus (from the chemical point of view) becomes a chain of ferment actions. Another most significant advance in animal physiology has been the sequel (as it were) of Bernard’s discovery of the formation of glycogen in the liver, a substance not to be excreted, but to be taken up by the blood and lymph, and in many ways more important than the more obvious formation of bile which is thrown out of the gland into the alimentary canal. It has been discovered that many glands, such as the kidney and pancreas and the ductless glands, the suprarenals, thyroid, and others, secrete indispensable products into the blood and lymph. Hence myxœdema, exophthalmic goitre, Addison’s disease, and other disorders have been traced to a deficiency or excess of internal secretions from glands formerly regarded as interesting but unimportant vestigial structures. From these glands have in consequence been extracted remarkable substances on which their peculiar activity depends. From the suprarenals a substance has been extracted which causes activity of all those structures which the sympathetic nerve-system can excite to action; the thyroid yields a substance which influences the growth of the skin, hair, bones, &c.; the pituitary gland, an extract which is a specific urinary stimulant. Quite lately the mammalian ovary has been shown by Starling to yield a secretion which influences the state of nutrition of the uterus and mammæ. A great deal more might be said here on topics such as these—topics of almost infinite importance; but the fact is that the mere enumeration of the most important lines of progress in any one science would occupy many pages.
Fig. 33.
The continuity of the protoplasm
of neighbouring vegetable cells, by
means of threads which perforate the
cell-walls. Drawing (after Gardiner)
of cells from the pulvinus of Robinia.