Some of the prettiest methods of demonstrating this process depend, not on the manufacture of starch in the leaf, but on the fact that an assimilating plant sets free oxygen, by breaking up the molecule CO2, building the carbon (C) into its own tissues, and letting the oxygen (O) go free. A beautiful method was discovered on these lines by
Engelmann, which I was never tired of seeing year after year in my Cambridge class. Defibrinated bullock’s blood is freed from air by means of an air pump and charged with CO2. In the course of this process it acquires the dingy tint of venous blood. A single leaf of the American weed (Elodea) is mounted on a glass slide in a drop of this blood and covered by an ordinary cover slip. Then comes the dramatic moment: the preparation is exposed to sunshine, and in 3 or 4 minutes a delicate scarlet border begins to appear round the leaf and grows rapidly, making a curious sunset effect in contrast with dingy purple of the venous blood. The meaning is very clear; the Elodea leaf in sunshine took the carbon from the CO2, and the oxygen thus set free gave the venous blood the scarlet hue characteristic of the arterial condition. Professor Farmer has designed a striking method based on another well-known experiment of Engelmann’s. A drop of water containing the products of decay, and therefore swarming with bacteria, supplies the test. A drop of this fluid is placed on a glass slip, one or two delicate leaves of a green water plant (Elodea) are added, and a square of thin glass is placed on it. Round the edges of the cover-slip the preparation must be sealed with a preparation of wax, which melts at a low temperature, and when cold serves to prevent the preparation drying; it also isolates it from the surrounding atmosphere. After making sure under the microscope that the bacteria are in active movement, the glass slip is placed in the dark for some 3 or 4 hours. It is then examined, and the bacteria will be found to have
ceased to move because they and the leaves between them have consumed the oxygen dissolved in the water, and bacterial activity being dependent on oxygen naturally came to an end. The preparation is placed under the microscope and illumined with bright incandescent gas, and after a short time the bacteria begin to stir and are soon once more whirling in their insensate dance. The reason is obvious—the green leaves under the influence of light were able to seize the carbon from the CO2, and the O thus set free put the bacteria in motion. The bacterial dance is therefore evidence of the act of assimilation carried on by the Elodea leaf.
Yet another method is worth mention, viz., that of Boussingault. The plant is placed in an inverted glass vessel resting in a dish of water, and is filled with hydrogen mixed with a percentage of CO2. Inside the vessel a fragment of phosphorus is suspended, and as a small amount of oxygen is sure to be mixed with the hydrogen the phosphorus will be oxygenated and white fumes will fill the vessel. The observer must wait until these clouds have subsided, which may need a couple of hours. This must take place in the dark, and as soon as the atmosphere is clear, the whole preparation is placed in bright light, when obvious clouds will again appear—a proof that oxygen has been set free by the assimilation of the green plants. With this example I must bring my short series of experiments to a close, with the hope that my readers may not deny that they are picturesque.
XIV
DOGS AND DOG LOVERS
“The more I see of men, the more I like dogs.”—Archbishop Whately. [219]
Why is it that some people do not like dogs? There are those who dislike other people’s dogs just as they dislike strange children. This is a point of view which is comprehensible though unattractive. Still, in comparison with those who do not like dogs at all this class seem positively amiable. I knew a lady with the most perfect understanding of the qualities of human beings, whether bad or good, yet she had no sympathy with dogs. She would be kind to them, as an external duty to all living things, but a dog had absolutely no place in her heart. What made this blindness seem all the more incomprehensible was the fact that she could love a bullfinch; she could not therefore plead that she loved humanity so much that she had no love left for beings of another sort. After all, it may be that not to care for dogs is no more a blemish than a lack of musical ear, which is not a sign of general dullness of artistic perception since it is found in some poets. We must accordingly allow that not to love dogs is not a sign of a black heart or
a debased nature. A dog lover will grant this to be an unavoidable intellectual conclusion, but in the secret corners of his mind he will feel something more hostile than mere Christian pity for these emotionally deformed people. If he holds Erewhonian doctrines he would like to send for the family straightener, and bear with fortitude the punishment inflicted on his friends and relations.
I fear that we, the dog lovers, are, by those who do not share our tastes, held to be unbalanced persons, who intrude their passions on the reasonable and well bred. They object to us as victims of perverted instincts, who talk unknown dog-language in and out of season. It is not clear to me why we care so much for dogs. Is it, in truth, an exaggeration, or an offshoot of that love of the helpless young of our own kind which natural selection develops in social animals? This is not necessarily maternal, as we see in the story of the heroic male baboon, who risked his life in saving a young one from a pack of baying hounds. [220a] Or is it an instinct developed in a hunting tribe—a blind tendency to take good care of the food-providers (at the expense of starving aunts and grandmothers), such as we see among the Fuegians, who explained that, “Doggies catch otters—old women no.” [220b]
However this may be, it is I think certain that the love of dogs is an unreasoning passion, having all the force of an instinct. In a story by Miss