The living cycads

 
9780217122498: The living cycads

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1919 Excerpt: ...of the Paleozoic age. The section of the leaf shows a structure which affords effective protection against the hot, dry weather of most cycad localities (Fig. 29). The highly cutinized surface is almost impervious to water, and the stomata, or pores, on the under side of the leaf have a form well adapted to keeping the opening closed except when the air is full of moisture. Long, narrow cells with thick, Fig. 30.--Dioon spinulosum at Tierra Blanca, Mexico; plants growing on rocks with little soil, showing exposed roots; at the right the base of a trunk with an exposed root (+) nearly as large as the trunk itself. corky walls are abundant, and they give the leaf a maximum of rigidity combined with elasticity. THE ROOT No extensive study of the cycad root has been undertaken. There is a main tap root which tapers gradually and extends down to a great depth, so that small amounts of water are brought up even when the surface soil is very dry. Lateral branches are usually small and weak. When plants are growing on exposed rocks where there is little soil, the root development is remarkable (Fig. 30). In the specimen shown in the figure the upper part of the roots is entirely exposed. Sometimes such roots may extend along the exposed rock for many feet before entering the soil. In an extreme case a root was exposed for forty feet before it finally entered the soil. A peculiar feature of the root has attracted much more attention and investigation than the normal structure itself. The small, lateral roots often become infected by "bacterioids" which cause a swelling accompanied by a profuse dichotomous branching, and besides the roots begin to grow up instead of down, soon emerging above the surface of the soil in dense coralloid masses (Fig. 31). After...

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