Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/79

Rh LIVERWORTS.] MUSCINEJE 67 forms between these two. Fig. 1 represents an example of a thallose Liverwort, showing the anterior portion of a plant of Mctzgeria furcata. On the right hand is shown the upper, on the left the under side. The vegetative body is a ribbon-shaped thallus, on the under side of which the clasping-roots (rhizoids) spring out of the mid-rib. Here the mid-rib is sharply separated from the side portions, but in other cases (as Ancura and Pellia) it passes into them by a very gradual transition. In front, in an indentation of the growing point (s), are found the only organs which are formed upon it. These are hairs secreting mucilage (papillae), and which stand on the under side. The mucilage which they secrete is prob ably intended for the purpose of protecting the vegetative point against the effects of drought. A peculiar exception appears in the growth of the genus Riclla (fig. 2). In this case a membranous wing, about one-fifth of an inch in width, of the finest green and of extreme delicacy, turns regularly on a central rib or axis, so as to form with the axis a sort of screw or winding staircase, having the form of an inverted cone with the apex down wards. The length of the whole plant of Riella helicophylla is about 2 inches. This plant, which is decidedly one of the most remarkable forms in the vegetable kingdom, grows upright in water, and was discovered in Algiers. At its lower end it is attached to the ground by clasping-roots. This is the only thallose Liverwort that grows upright, and also one of the few that grow in water. There are likewise two species of Riccia which swim in water Riccia natans and Riccia fluitans ; but these also possess forms which live on land, and it is these forms which attain to the formation of sporogonia. It follows hence that these plants also were originally in every case land plants. A few others too, such as Pellia epiphylla and Marchantia polymorpha, have the power of vegetating in water, but this happens only exceptionally, and the plants are then sterile. Returning to the thallose Liverworts that live on land, we have to remark that the differentia tion of their tissues is a very simple one. The cells of the thallus, so far as they are not em ployed in the formation of clasping-roots or ofpio. 2. Plant of sexual organs, are all homogeneous. Those only Riella helicophylla, of the mid-rib present a difference in Metzgeria, from A1 S iers - Blyttia, and others by their lengthened form. From this simple form has arisen a further complication in the development of the vegetative body in two different series. In the Marchantiaceie the vegetative body retains the form of a thallus, but its anatomical organization is of higher structure. In the Jungcrmanniacete it is the external organization of the vegetative body which is the richer, and here the transition takes place in it to the foliose Liverworts, which possess a stem with leaves. We may consider as representative of the Marchantiaceie the very widely - spread Marchantia poly morpha. Its ve getative body is a broad ribbon- shaped thallus, which has on its under side clasp ing-roots (single- celled tubules, which are either simple, or have on their inner side characteristic coni cal or tubercular thickenings) and two rows of mem branous lamellae, the uppermost of which cover the growing point and FlG- 3- _ (1) Longitudinal section through the growing- lorm a protection point (apex) of Fegatella c/mica. On the upper side of the thallus are formed the air-chambers, as depres sions in the upper surface K. On the under side of the thallus are formed membranous scales L, which always proceed from one cell. (2) Surface -view of a young portion of the thallus ; the stomata appear as gaps in the epidermis. They are surrounded by a few concentric, cells, of which the first are represented. (3)i (4), (5) Marcliantia polymorpha ; development of the air-chambers and stomata. In (5) a number of cell- threads containing chlorophyll are seen sprouting from the bottom of the air-chamber. to it. (See the section through Fegatella, an allied form, fig. 3,(1),Z.) The upper surface presents a division into small rhom- boidal spaces (ar- eolre), and in the middle of each space is an opening, the stoma or breathing hole. A section through the thallus shows a strongly-marked epidermis broken through at definite spots by the stomata or breathing holes, which, however, in this case have a very different structure from that presented in the higher plants. In Marchantia (fig. 3, (5), Sp) they are of barrel-shaped figure, and consist of several rings or tiers of cells placed over one another. Their origin may be traced in fig. 3, (3) and (4). Below the epidermis we find chambers filled with air. These, however, are not empty ; from their base sprout forth cellular threads (fig. 3, (5)), the cells of which contain a large quantity of chlorophyll. It is in these that the assimilation of the carbon takes place. Those cells of the thallus which lie beneath serve only for the storing up and transmission of the materials for construction. The individual chambers are separated from each other by flat layers of cells, and on these rest the rhomboidal areolse of the upper surface which can be seen with the naked eye. In other Marchanticse the stomata are of a different structure. In Fegatella, for example, they are not barrel-shaped, but are bounded by several concentric circles of cells, which all lie on the upper surface of the thallus. Early stages of these are repre sented in fig. 3, (2). The way in which the air-chambers and stomata attain their condition is very remarkable. Small depressions in the cell-layer of the upper surface are formed in large numbers behind the growing point (fig. 3, (1), U). These widen considerably, and become, as it were, roofed over by the neighbouring parts, so that an epidermis (fig. 3, (1), E) grows in this manner over the hol lows from the bottom of which at a later period the green cellular threads sprout forth. The strings of mucilage-cells which are found in the tissue of Fegatella, the brown threads which appear in that of Prcissia, and the cells filled with a brown oily secretion which are found in Marchantia and other species must also be mentioned. The second series, which comes next to the simple thallose forms, is that of the foliose or leafy Jungcrmannic&. Forms intermediate between the thallose and the foliose are represented by the species Blasia and Fossomb-ronia. The former, which not unfrequently grows in the damp soil of forests, possesses a flat, thallus-like stem, which bears on its longitudinal axis two rows of leaves inserted parallel to each other, and has also on the under side two rows of toothed scales, the amphigastria. Fossombronia makes a nearer approach to the proper leafy forms ; it possesses two rows of leaves inserted obliquely to the longitudinal axis of the flattened stem. In the foliose Jungermannie&, in the narrower sense of the term, the stalk is thin and thread-like, and bears distinctly defined leaves. In this case also with the exception of the upright growing Haplomitrium Hookeri the stem grows in close connexion with the substratum. The leaves stand in three rows : two lateral, and one ventral towards the substratum. The ventral leaves are in this case also, as in Blasia, termed amphigastria or under leaves. They are smaller than the side leaves, and are sometimes reduced to the form of mere hairs, or even are entirely wanting, as in Jimgermannia bicuspidata, where it is only in rare exceptional cases that they appear. The leaves of Liverworts consist of a single layer of cells without any mid-rib. They are commonly bifid at the apex, but in many forms this condition is only recognizable in early development, and disappears before maturity. In general the leaves stand so thickly that they overlap each other with their margins, and quite cover the upper surface of the stem. The way in which the leaves overlap has been employed as a systematic character. Overshot leaves (folia succuba) are those in which the anterior margin turned towards the vegetative point of the stem stands higher than the posterior one, and thus the anterior margin of each leaf overlaps the posterior margin of the leaf which stands before it, while its own posterior margin is overlapped by the anterior margin of the leaf which stands behind it. In the opposite case of undershot leaves (folia incuba), the posterior margin of the leaf is higher than its anterior margin, and the anterior margin, inclining obliquely outwards, is overlapped by the posterior margin of the leaf which comes next before it, as in fig. 4. The mode in which the leaves overlap each other depends on a difference in the relative amount of growth of the upper or dorsal and the under or ventral side respectively. If the growth of the upper side preponderates, then we have the overshot, in the opposite case the undershot mode of covering. Originally the leaves do not stand obliquely to the longitudinal axis ; it is only gradually that they are shifted out of their place. The position of the leaves is explained by their origin. The apex of the stem is occupied by a cell which has the form of a three -sided pyramid with a vaulted base. The surface of one side of the pyramid is directed downwards, the two others are directed laterally. This apical cell is divided by walls which are parallel alternately to one of the side walls. Thus seg ments are cut off from the apical cell and lie in three rows. From each of these segments n rises a leaf. The leaves are thus necessarily likewise arranged in three rows, one of which is turned towards the substratum. The anatomical structure of the stem it also a very simple one. Its exterior cells have their walls somewhat more strongly thickened, and thus form a firmer rind-layer. We must also notice the secre tion of dark-brown oily corpuscles, which undergo no further con version, and are met with in certain leaf-cells. In the thalloso