Fig. 520. Plant of Marsilia quadrifoliata, reduced in size; at the right a pair of sporocarps of about natural size.
[495.] The Pillworts (Marsilia and Pilularia) are low aquatics, which bear globular or pill-shaped fruit (Sporocarps) on the lower part of their leaf-stalks or on their slender creeping stems. The leaves of the commoner species of Marsilia might be taken for four-leaved Clover. (See Fig. [520].) The sporocarps are usually raised on a short stalk. Within they are divided lengthwise by a partition, and then crosswise by several partitions. These partitions bear numerous delicate sacs or spore-cases of two kinds, intermixed. The larger ones contain each a large spore, or macrospore; the smaller contain numerous microspores, immersed in mucilage. At maturity the fruit bursts or splits open at top, and the two kinds of spores are discharged. The large ones in germination produce a small prothallus; upon which the contents of the microspores act in the same way as in Ferns, and with a similar result.
[496.] Azolla is a little floating plant, looking like a small Liverwort or Moss. Its branches are covered with minute and scale-shaped leaves. On the under side of the branches are found egg-shaped thin-walled sporocarps of two kinds. The small ones open across and discharge microspores; the larger burst irregularly, and bring to view globose spore-cases, attached to the bottom of the sporocarp by a slender stalk. These delicate spore-cases burst and set free about four macrospores, which are fertilized at germination, in the manner of the Pillworts and Quillworts. (See Fig. [521-526].)
Fig. 521. Small plant of Azolla Caroliniana. 522. Portion magnified, showing the two kinds of sporocarp; the small ones contain microspores. 523 represents one more magnified. 524. The larger sporocarp more magnified. 525. Same more magnified and burst open, showing stalked spore-cases. 526. Two of the latter highly magnified; one of them bursting shows four contained macrospores; between the two, three of these spores highly magnified.
[497.] Cellular Cryptogams ([483]) are so called because composed, even in their higher forms, of cellular tissue only, without proper wood-cells or vessels. Many of the lower kinds are mere plates, or ribbons, or simple rows of cells, or even single cells. But their highest orders follow the plan of Ferns and phanerogamous plants in having stem and leaves for their upward growth, and commonly roots, or at least rootlets, to attach them to the soil, or to trunks, or to other bodies on which they grow. Plants of this grade are chiefly Mosses. So as a whole they take the name of
[498.] Bryophyta, Bryophytes in English form, Bryum being the Greek name of a Moss. These plants are of two principal kinds: true Mosses (Musci, which is their Latin name in the plural); and Hepatic Mosses, or Liverworts (Hepaticæ).