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of an inch in diameter, others being as much as iody or They often form the centre of the grains of chlorophyll, as Mohl has shown. In the milky juice of Euphorbia, they assume the singular appearance represented at Plate II. fig. 19. b., looking like short cylinders enlarged at each end into a round head: double-headed granules of this kind are not as yet found elsewhere; Morren states that they vary in form in different species of Euphorbia.
Their nature has been carefully investigated by Fritzsche (Ueber das Amylum, Berlin, 1834, and Poggendorf's Annalen, 1834, No.9, 10.), who has proved them to be formed by the successive deposit of new layers, one over the other, and not to be cells containing soluble matter, as Raspail asserts.
Those which have the smallest size have a distinct motion of rotation when suspended in water; and this motion looks as if spontaneous ; for of several floating near each other, in the same medium, a part will be in active motion, while others remain inactive.
Tarpin calls these granules Globuline, and considers them the most elementary conditions of vegetable tissue, its primitive form; an opinion which is adopted, with some modifications, by Raspail, who looks upon each granule as one of the elementary
molecules of tissue in a state of development. This writer assigns them a point of attachment or hilum, by which they originally adhered to the parent cell : he considers that cellular tissue is produced by the development and mutual pressure of each granule, and that all the varied forms of plants may be explained by reference to this principle. (Nouv. Syst. de Chimie Organique, p. 83.) Morren states that these grains of fecula are the first stage of a crowd of organs, and that he can demonstrate the free spiral thread of Collomia and Salvia to be at first an amylaceous granule. This, however, does not correspond with the statements of Schleiden. Such amylaceous granules appear to have, under particular circumstances, the power of spontaneous growth, by which they multiply and increase themselves externally. This is particularly visible in the fecula of Barley; which, if observed in its original state, is found to be composed of angular, irregular bodies, some of which are of extreme minuteness, and seem
to have a power of spontaneous motion in water. Shortly after germination the amylaceous bodies, according to Turpin, appear to lose their substance, to become more transparent and flaccid, a circumstance which he thinks is owing to the chemical change of their starch into sugar: the granules however at this time retain their property of becoming blue under the action of iodine. When this alteration has been produced, the maltster stops the new chemical action by heat and dryness, and fixes the sugar, as we see in malt. When the amylaceous granules are placed in water of a certain temperature, rendered sweet by the dissolution of their own sugar, and exposed to the influence of the oxygen of the atmosphere, they evidently produce little sprouts like themselves from their sides. Turpin states that, if examined after fermentation has been going on for some hours, they will be found to have each formed several new granules exactly like the mother-granules; and he not only considers this to be the cause of the curious phenomena observable in fermentation, but regards the granules as seeds and the result of their growth as a plant, which he calls Torula cerevisia. He adds, that in the inside of each of the new granules formed during fermentation, he finds a number of still smaller granules. I have not repeated the observations of this ingenious writer further than to ascertain that the granules in fermentation do sprout; and that they have at that time lost all their starch, for iodine produces no sensible effect upon their colour; a circumstance to which he has not adverted.
According to Schleiden amylaceous granules are gradually changed into gum and mucus in the process of lignification. (Beiträge zur Phytogenesis, p. 17.) This author considers starch to be analogous in plants to the fat in animals. It is nutritious matter in excess, laid by for future use, and is usually found in those places where new organization is about to commence, or where a luxuriance of vegetation has produced an excess of nutriment. Starch is sometimes represented by another half-granular matter, found in pollen, in the albumen of some plants, and abundantly in the Parenchyma of leaves, as the centre of the Chlorophyll. It is especially distinguished by presenting itself in irregular granular bodies without any internal structure, and becomes brownish yellow or brown when tested with tincture of iodine. This may be called mucus, and appears to be what the Cytoblast and its spherule are composed of. When starch is about to assume a new organization, it converts itself, in some manner unknown to chemists, into sugar or gum.
Sugar makes its appearance as a transparent fluid, which seems as clear as water, is not rendered turbid by alcohol, and is coloured by tincture of iodine, according to the greater or less degree of dilution of that agent.
Gum appears as a yellowish, more consistent, less transparent fluid, which, with tincture of iodine, coagulates into a pale yellow ungranulated colour. When vegetation has advanced to that point that gum is the latest immediate product, there appears in it a great many minute molecules, which are generally so small as to resemble dark points; at that tiine the fluid becomes a darker yellow upon the application of iodine. But the molecules, if they are large enough to show their colour, become dark-brown yellow. It is this mass, so transparent that it can hardly be seen till it is coloured, in which, in all cases, organization commences, and from which the youngest structure is constituted. It may be called Vegetable Jelly, and is probably nearly the same as Pecten, the base of Gum Tragacanth, and many other kinds of vegetable mucus. It is this jelly, which, by a further chemical attraction, becomes the membrane of cells, and is afterwards the material by which it is thickened.
2. Chlorophyll or Chromule. - To this is referred all the kinds of coloured granules which occupy the interior of vegetable tissue. They have a spheroidal, irregular figure, are often rather angular, consist of a semi-fluid gelatinous substance not contained in a sac, and which seems to be a coagulum of the fluid contents of the cells. The colour of plants, especially the green colour, is produced by the presence of chlorophyll, which may be considered a vital secretion. It will be mentioned more particularly in Book II., in the chapter upon colour.
3. Later granules. — The interior of the laticiferous vessels is occupied by a fluid rendered turbid by the presence of infinite quantities of extremely minute particles which do not become blue upon the application of iodine, but which have a rapid motion upon their own axis. Of these little more is known than that they exist.
OF THE COMPOUND ORGANS IN FLOWERING PLANTS.
Having now explained the more important circumstances connected with modifications in the elementary organs of vegetation, the next subject of enquiry will be the manner in which they are combined into those masses which constitute the external or compound organs, or in other words the parts which present themselves to us under the form of roots, stems, leaves, flowers, and fruit, and which constitute the
apparatus performing all the actions of vegetable life. In doing this, I shall limit myself in the first place to Flowering Plants (Introduction to the Natural System, p. 1.); reserving for the subject of a separate chapter the explanation of some of the compound organs of Flowerless plants (ibid. p. 395.), which differ so much in structure from all others, as to require in most cases a special and distinct notice.
Sect. I. Of the Cuticle and its Appendages.
1. Of the Epidermis.
VEGETABLES, like animals, are covered externally by a thin membrane or epidermis, which usually adheres firmly to the cellular substance beneath it. To the naked eye it appears like a transparent homogeneous skin, but under the microscope it is found to be traversed in various directions by lines, which, by constantly anastomosing, give it a reticulated character. In some of the lower tribes of plants, consisting entirely of cellular tissue, it is not distinguishable, but in all others it is to be found upon every part exposed to the air, except the stigma and the spongelets of the roots. It is, however, as constantly absent from the surface of parts which