. An introduction to the structure and reproduction of plants. Plant anatomy; Plants. STRUCTURE AND DEVELOPMENT OF SPORANGIA 297 A mature sporangium (cf. also Fig. 166, B) consists of a stalk of usuall}' three rows of elongated cells, terminated by a biconvex capsule which is more or less oval in side-^'iew, and encloses the spores within a wall of a single la\'er of cells. The cells of the wall fit firmly together, and most of them are thin-walled and \'ery flat, appearing more or less polygonal in surface-view, but tabular in optical section. The edge of the capsule, how- e^•cr, is occupied
. An introduction to the structure and reproduction of plants. Plant anatomy; Plants. STRUCTURE AND DEVELOPMENT OF SPORANGIA 297 A mature sporangium (cf. also Fig. 166, B) consists of a stalk of usuall}' three rows of elongated cells, terminated by a biconvex capsule which is more or less oval in side-^'iew, and encloses the spores within a wall of a single la\'er of cells. The cells of the wall fit firmly together, and most of them are thin-walled and \'ery flat, appearing more or less polygonal in surface-view, but tabular in optical section. The edge of the capsule, how- e^•cr, is occupied by a single row of speciallv differentiated cells. The greater part of this band, from the base of the capsule on one side to half-waj' down the other edge, is com- posed of cells which have all but their outer walls strongly thickened, and which constitute the annuliis {a. in Fig. 165 and an. in Fig. 166, B). The re-. mainder, forming the so-called' siomium, are much broader and thin-walled throughout (Fig. 166, B, s.), and it is here that the mature capsule ruptures. Each sporangium arises as a papillate outgrowth from a single surface cell of the placenta, which is cut off bj' a cross- wall, and undergoes division by- three oblique septa to produce a tetrahedral apical cell (b^ig. 166, C, I, II). The three rows of segments, primaril)- cut off parallel to the three flat faces of the latter (III), elongate and become the stalk. Next, walls are formed parallel to all four faces of the apical cell (IV), and give rise to the one-la3-ered capsule-wall (Fig. 166, D and E, u\), an inner nutritive laj-er, or tapdiim {(.), and a central cell, the archesporiimi {a.). The last-named divides to form the spoix mother-cells, each of ^^hich gives rise to four spores (cf. p. 309). Their development takes place at the expense of the food-materials contained in the Fig. 164.—On the left part of a Filmy Fern {Hviuenophvlliiin tiiiibyidgeiise), bearing sori (s.). The two smaller figures on t
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectpl, booksubjectplants
