. An introduction to vegetable physiology. Plant physiology. 22 VEGETABLE PHYSIOLOGY Yet a third reason may be given. The leaves are very frequently so placed that they extend outwards from the plant and lie nearly parallel to the surface of the ground. In this way they present their edges to the wind and offer as little obstacle as possible to its passage through the tree, so minimising the risk of being torn off when the force of the wind is strong. As the wind passes between them they are made to rise and fall, but they offer much less resistance to its force than they woiild if they were n
. An introduction to vegetable physiology. Plant physiology. 22 VEGETABLE PHYSIOLOGY Yet a third reason may be given. The leaves are very frequently so placed that they extend outwards from the plant and lie nearly parallel to the surface of the ground. In this way they present their edges to the wind and offer as little obstacle as possible to its passage through the tree, so minimising the risk of being torn off when the force of the wind is strong. As the wind passes between them they are made to rise and fall, but they offer much less resistance to its force than they woiild if they were not flattened. Besides the plants which we have beea examining there are other forms of terrestrial habits which possess only weak axes, quite incapable of supporting any great development of their shoot system. These obtain support by clinging in various ways and holding by various mechan- isms to other structures, such as the trunks of trees, walls, &e. In some cases they develop accessory root systems from some part of their shoots, such roots, usually of small dimensions, penetrating their supports and so securing anchorage. The first indication of structural differentiation in the vegetative body of the plant is a change in the character of the exterior, which has for its object the protection of the plant from external injurious influences. This can be seen even among the seaweeds, simple as is generally the structure of members of this group. Fucus and its aUies, which form part "" w^;''"" mZ of the class of the brown Alg^, have SHOWING chaeactee o! their oxtemal cells much smaller, ^cZ^Tsoa" ^«^« do^ely P^t together, and generally much denser than the rest of their tissue (fig. 19). In the group of the Mosses certain arrangements of this kind can be seen. The common bog moss {Sphagnum) shows its stem to have on the outside several layers of large empty cells. Please note that these images are extracted from scanned page images that may have
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