. An introduction to the structure and reproduction of plants. Plant anatomy; Plants. 3ib SPORES OF SELAGINELLA These features are readily observed in longitudinal sections through the cones, which also show the presence of a small outgrowth (the ligule, Fig. i8i, /.) from the upper surface of each sporophyll, between its upturned tip and the sporangium. Such ligules, though most conspicuous on the sporophylls, oc- cur also on all the vegetative 1 e a ^f e s , but their function is altogether obscure. A ligule is not met with in the genus Lycopo- diiiin, but ap- pears to ha\'e been character-
. An introduction to the structure and reproduction of plants. Plant anatomy; Plants. 3ib SPORES OF SELAGINELLA These features are readily observed in longitudinal sections through the cones, which also show the presence of a small outgrowth (the ligule, Fig. i8i, /.) from the upper surface of each sporophyll, between its upturned tip and the sporangium. Such ligules, though most conspicuous on the sporophylls, oc- cur also on all the vegetative 1 e a ^f e s , but their function is altogether obscure. A ligule is not met with in the genus Lycopo- diiiin, but ap- pears to ha\'e been character- istic of most of the extinct re- presentativesof the f a m i 1 J', which also pos- sessed t^vo kinds of spores. The 3'oung sporangia of Selagindlahd^ye a several-lay- ered wall, en- closing a large numberofspore mother-cells, and, up to this stage, all are alike. In the nncro- sporangia each mother-cell gives rise to four small spores, but in the megasporangia only one develops further, enlarging rapidly at the expense of the others, and dividing to form the single tetrad (Fig. i8i, A). An American species (S. rupestris) exhibits an even greater reduction, since occasionally only one of the four megaspores reaches maturity, and, in this and. Fig i8i.—Selaginella tittibrosa. A, Longitudinal section tlirough part of a cone, sliowing micro- sporangia (mi.) above and megasporangia [mg.) below. E, Small part of megasporangium en- larged to show the stalk and wall. I., ligule. B, Single megaspore enlarged. C, Tetrad of microspores on the same scale of magnification as B. L), Tetrad of micrcisporcs Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Fritsch, Felix Eugene, 1879-; Salisbury, E. J. (Edward James), Sir, b. 1886. London, G. Bell and sons ltd.
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectpl, booksubjectplants
