. The elements of botany for beginners and for schools. Botany. SECTION 17.] PTERIDOFHYTES. 159 surface of the leaf or frond, or over the whole surface when there are no proper leaf-blades to the frond, but all is reduced to stalks. Commonly the spore-cases occupy only detached spots or lines, each of which is called a SoRUS, or in English merely a Fruit-dot. In many Ferns these fruit-dots are naked ; in others they are produced under a scale-like bit of membrane, called an Indusium. In Maidenhair-Ferns a little lobe of the leaf is folded back over each fruit-dot, to serve as its shield or ind
. The elements of botany for beginners and for schools. Botany. SECTION 17.] PTERIDOFHYTES. 159 surface of the leaf or frond, or over the whole surface when there are no proper leaf-blades to the frond, but all is reduced to stalks. Commonly the spore-cases occupy only detached spots or lines, each of which is called a SoRUS, or in English merely a Fruit-dot. In many Ferns these fruit-dots are naked ; in others they are produced under a scale-like bit of membrane, called an Indusium. In Maidenhair-Ferns a little lobe of the leaf is folded back over each fruit-dot, to serve as its shield or indusium. In the true Brake or Bracken (Pteris) the whole edge of the fruit-bearing part of the leaf is folded back over it like a hem. 488. The form and structure of the spore-cases can be made out with a common hand magnifying glass. The commonest kind (shown in Fig. 503) iias a stalk formed of a row of jointed cells, and is itself composed of a layer of thin-walled cells, but is incompletely surrounded by a border of thicker-walled cells, forming the Ring. This extends from the stalk up one side of the spore-case, round its summit, descends on the other side, but there gradually vanishes. In ripening and drying the shrinking of the cells of the ring on the outer side causes it to straighten; in doing so it tears the spore-case open on the weaker side and discharges the minute spores that fill it, com- monly with a jerk which scatters tliem to the wind. Another kind of spore-case (Fig. 507) is stalkless, and has its ring-cells forming a kind of cap at the top: at ma- turity it splits from top to bottom by a regular deliiscence. A third kind is of firm texture and, opens across into two valves, like a clam-shell (Fig. 508") : this kind makes an approach to the next family. 489. The spores germi- nate on moistened ground. In a conservatory they may be found germinating on a damp wall or on the edges of a well-watered flower-pot. Instead of directly forming a fern-plautlet, th
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Keywords: ., bookpublishernewyorkamericanboo, booksubjectbotany, bookyear1887
