. An introduction to vegetable physiology. Plant physiology. 44 VEGETABLE PHYSIOLOGY no definite aggregation of them into micellae. He pictures, therefore, a linkage of the atoms into a molecular network, the meshes of which are occupied by water. On either hypothesis the quantity of water is capable of considerable increase or diminution, and the wall can be made to swell up by causing it to imbibe more fluid. This can be brought about by exposing it to the action of strong mineral acids, such as sulphuric acid. The water is held, however, by the solid particles with very great tenacity. The
. An introduction to vegetable physiology. Plant physiology. 44 VEGETABLE PHYSIOLOGY no definite aggregation of them into micellae. He pictures, therefore, a linkage of the atoms into a molecular network, the meshes of which are occupied by water. On either hypothesis the quantity of water is capable of considerable increase or diminution, and the wall can be made to swell up by causing it to imbibe more fluid. This can be brought about by exposing it to the action of strong mineral acids, such as sulphuric acid. The water is held, however, by the solid particles with very great tenacity. The thickening which always supervenes to a greater or less extent upon the first formation of the cell-wall is brought about by the protoplasm in a way similar to that of its original construction. Layers composed like the original one are continually secreted by the protoplast and are deposited upon its exterior in apposition with the wall already there. Hence walls which have a per- ceptible thickness show a certain stratification, which is most easily seen in transverse sections (fig. 42). When several such layers can be distinguished it has been found that pectic bodies are prominent in the layers furthest from the protoplasm and cellulose in those nearest the interior of the cell. The action of the proto- plast is frequently irregular, so that the thickening layers are often seen as bands of various form, giving the surface of the membrane particular patterns, thin and thick places alternating in various ways (fig. 43). These are seen most conspicuously in the walls of the vessels of the wood. In some cases the thickening is caused or materially aided by the intercalation of fresh molecules of cellulose into the substance of the existing wall. This process is known as intussusception. It appears to be not so general as was formerly Fig. 42.—Thickened Cells of Wood, showing Stratifica- tion. (After Sachs.). Please note that these images are extracted from scanned
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