قراءة كتاب More Science From an Easy Chair

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More Science From an Easy Chair

More Science From an Easy Chair

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دار النشر: Project Gutenberg
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carried in a wooden tank full of water, with a close-fitting lid to prevent their jumping out. I saw him take a seventh. The largest must have weighed nearly two pounds. It seems almost incredible that fish should inhabit water so cold, so opaque, and so torrential, and should find there any kind of nourishment. They make their way up by keeping close to the bank, and are able, even in that milky current, to perceive and snatch the unfortunate worm or grub which has been washed into the flood and is being hurried along at headlong speed. Only the trout has the courage, strength, and love of nearly freezing water necessary for such a life—no other fish ventures into such conditions. Trout are actually caught in some mountain pools at a height of 8,000 ft., edged by perpetual snow.

You are rarely given trout to eat here in the hotels. A lake fish, called "ferras," a large species of the salmonid genus Coregonus, to which the skelly, powan, and vendayce of British lakes belong, is the commonest fish of the table d'hôte, and not very good. A better one is the perch-pike or zander. It is common in all the larger shallow lakes of Central Europe, and abounds in the "broads" which extend from Potsdam to Hamburg, though it is unknown in the British Isles. It is quite the best of the European fresh-water fish for the table, and there should be no difficulty about introducing it into the Norfolk Broads. It would be worth an effort on the part of the Board of Agriculture and Fisheries to do so, as the perch-pike, unlike other fresh-water fishes, would hold its own on the market against haddock, brill, and plaice. Another interesting fresh-water fish which grows to a large size in the Lake of Geneva (where I have seen it netted) is the burbot—called "lote" in French—a true cod of fresh-water habit which, though common throughout Europe and Northern Asia, is, in our country, only taken in a few rivers opening on the east coast. It is a brilliantly coloured fish, orange-brown, mottled with black, and is very good eating.

Passing up the Lauterbrünnen valley, I came upon some wild raspberries and quantities of the fine, large-flowered sage, Salvia glutinosa, with its yellow flowers, in shape like those of the dead-nettle, but much bigger. They were being visited by humble-bees, and I was able to see the effective mechanism at work by which the bee's body is dusted with the pollen of the flower. I have illustrated this in some drawings (Fig. 1) which are accompanied by a detailed explanation. Two long stamens, a1, arch high up over the lip of the flower, li, on which the bee alights, and are protected by a keel or hood of the corolla. Each stamen is provided with a broad process, a2, standing out low down on its arched stalk, and blocking the way to the nectar in the cup of the flower. When the bee pushes his head against these obstacles and forces them backwards, the result is to swing the long arched stalk, with its pollen sacks, in the opposite direction, namely, forwards and downwards on to the bee's back. It was easy to see this movement going on, and the consequent dusting of the bee's back with pollen. In somewhat older flowers, which have been relieved of their pollen, the style, st., or free stalk-like extremity of the egg-holding capsule, already as long as the stamens, grows longer and bends down towards the lip or landing-place of the yellow flower. When a pollen-dusted bee alights on one of these maturer flowers the sticky end of the now depending style is gently rubbed by the bee's back and smeared with a few pollen-grains brought by the bee from a distant flower. These rapidly expand into "pollen tubes," or filaments, and, penetrating the long style, reach the egg-germs below. Thus cross-fertilization is brought about by the bees which come for the nectar of Salvia. The stalks and outer parts of the flower of this plant produce a very sticky secretion which effectually prevents any small insects from crawling up and helping themselves to the nectar exclusively provided for the attraction of the humble-bee, whose services are indispensable.

Fig. 1.—Diagrams of the flower of the yellow sage (Salvia glutinosa) a little larger than life. 1. An entire flower seen from the side. st. The stigma, a2. The pair of modified half-anthers which are pushed back by the bee when inserting its head into the narrow part of the flower. 2. A similar flower at a later stage when the stigma, st., has grown downwards so as to touch the back of a bee alighting on the lip of the flower, and gather pollen from it. 3. Diagram of one of the two stamens. f. The stalk or filament of the stamen. a1. The pollen-producing half-anther, eo. The elongated connective joining it to the sterile half-anther. 4. Section through a flower showing ov. the ovary; nec. the nectary or honey-glands; st. the style; li. the lip of the flower on which the bee alights. 5. Similar section showing the effect of the pushing back of a2 by the bee, and the downward swinging of the polliniferous half-anther so as to dust the bee's back with pollen. The dotted arrow shows the direction of the push given by the bee.

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