Nature and development of plants . phyte and become the dominant gene-ration in the life history of the plant. Order 2. Filicales or Common Ferns 113. General Features.—The great majority of plants popu-larly known as ferns belong to this order. They are a highlyspecialized group that have branched off from the primitive fernsin recent geological times and owing to their variations beinghighly adaptive to present conditions upon the earth, they havebecome very numerous and widely distributed. In temperateclimates the majority live upon the ground in moist and shadyregions and some are aquatic
Nature and development of plants . phyte and become the dominant gene-ration in the life history of the plant. Order 2. Filicales or Common Ferns 113. General Features.—The great majority of plants popu-larly known as ferns belong to this order. They are a highlyspecialized group that have branched off from the primitive fernsin recent geological times and owing to their variations beinghighly adaptive to present conditions upon the earth, they havebecome very numerous and widely distributed. In temperateclimates the majority live upon the ground in moist and shadyregions and some are aquatic or xerophytic. They attain theirgreatest abundance in the mountainous district of tropical coun-tries where they occur in astonishing profusion and variety uponthe moist rocks and trunks of trees, as well as upon the earth. 292 NATURE OF THE FILICALES The leaves are the most striking feature of the Filicales. Theyare usually large and divided and are characterized by beingcoiled when young (Fig. 219). This is due to the stronger. Fig. 219. Christmas fern, Polystichum, with prostrate stem bearing atthe tip young coiled leaves covered with chaffy scales and further backlarge leaves of the previous season. The older portions of the stem arecovered with the petioles of dead leaves.—H. O. Hanson. growth of the outer cells which causes an inward rolling of theleaf. The growth of the leaf is very slow, often requiring threeyears for its formation in temperate regions. During the seasonpreceding its expansion, the petiole and blade are completelyformed and appear in crosier-like coils more or less covered withchaffy scales (Fig. 220, r). This development enables the leavesto expand with surprising rapidity in the spring when the morerapid enlargement of the cells on the inner side of the leaf cause DEVELOPMENT OF PLANTS 293 it to uncoil. The tissues of the leaves are differentiated into anepidermis, stomata, chlorenchyma and vascular bundles as in thehigher plants (Fig. 223). The s
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