Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/81

Rh LIVERWORTS.] MUSCINE^E 69 fertilization a special goblet-shaped involucre, whilst every group of archegonia is also enveloped by two scales, which incline towards each other like the valves of a mussel-shell. Fin. 6. Marchantia polymorpha. Archegonia at different stages of develop ment, in longitudinal section. (1) An archegonium not yet open ; its neck portion is still covered at its point by the lid-cells d. In the ventral portion lies a large cell, which has divided into a small upper cell, the &quot;ventral canal-cell,&quot; b, and a lower one, the oosphere. In (2) the neck portion of the archegonium is open ; the oosphere has assumed a spherical form ; the ven tral canal-cell is shrivelled up. (3) A fertilized archegonium ; the oosphere has become developed into an embryo, here consisting of eight cells, d is an involucre, which in Marchantia grows round the archegonium after ferti lization. (After Sachs.) (3) Development of the Asexual Generation of the Sporogonium. Just as in the vegetative classification we are led in a continuous series from the simple thallose forms to the foliose forms, so in the perfecting of the fertilized oosphere into an embryo (here the sporo gonium) there is a gradual advance from simple to more complex forms. The first result of fertilization always is that the oosphere becomes surrounded by a membrane, grows, and transforms itself by division into a cellular body. In the simplest case, in Riccia (fig. 7, A), the only differentia tion which takes place in this cel lular body is that it forms a wall- layer which is soon reabsorbed, while the whole of the interior cells form spores in such a way that by division four spores pro ceed from every mother-cell. In Marchantia (fig. 7, B) we find a FIG. 7. Young embryos of Liverworts in longitudinal separation tak- section (semi -diagrammatic). The cells from the divi sion of which the mother-cells of the spores proceed are shaded. A. A spherical embryo of Riccia, without stalk. B. Marchantia polyinorpha ; the upper part of the em bryo Ka turns into the capsule, the lower part into the stelk. C. Pellia epiphylla ; the cell a forms an append age to the embryo ; the stalk proceeds from the cells marked by brackets, and from those above is formed the capsule, whose wall and internal mass are now distin guishable. The whole inner portion of the capsule pro ceeds from the division of four cells. D. Anthoceros Ixvis; col, columella. E. Jungermannia licuspidata. F. Radula complanata. E shows the arrangement of the cells of the embryo which is characteristic of the Jun- germanniex ; it is composed of a number of transverse disks, which always consist of four cells. ing place in the embryo, in which its inferior por tion (that be neath the line 1, 1) becomes the short stalk of the sporogonium and its superior por tion becomes the capsule. In the JungermannicaR (fig. 7, C, E, F) we find in the young embryo a group of cells from which proceed the mother -cells of the spores (shown by hatched _ lines in the figures) ; they are covered by a layer of cells which at a later period becomes the wall of the spore -cap sule. Lower down (distinguished by a bracket in fig. 7, C) conies that part of the embryo out &quot;of which the stalk of the sporogonium is formed, while the cell a represents an appendage of the embryo. The inferior portion of the stalk often swells into a thickened foot, which frequently penetrates deeply into the tissue of the fertile shoot (fig. 8). The upper part of the sporogonium, in which the spores and elaters are formed, swells into a sphere. In the Jungermanniese, when the sporogonium attains maturity, the stalk becomes very considerably lengthened, the ventral part of the archegonium, which had hitherto kept pace with it in growth, flies asunder, and the capsule emerges. This opens in various ways. In the Junger* mannieas the wall breaks up into four valves (fig. 4). The sporogonia of Anthoceros behave in a ^ &quot;. RB^^c no D MM err very peculiar manner. The period occupied by their development is much longer than that of the Jungermannicae. While they are dis charging mature spores at their apex they are still growing at their base, and forming new spores. The sporogon ium dehisces by two valves, and is traversed in the middle line by a longitudinal string of cells not employe the formation of spores. This persists till the maturation of the sporogonium as a little column, and is termed the &quot;columella&quot; (see ficr 7 T) TJio llo !. /: ;,, irom wnicn the mother- , FIG. S. Jungermannia bicuspidata. Longitudinal section of the immature sporogonium sg, sur rounded by ar, the calyptra (the ventral por tion of the archegonium which has kept pace with it in growth). In the sporogonium three parts are to be distinguished : the globular cap sule in which lie the mother-cells of the spores and the elaters, the cylindrical stalk which at a later period lengthens very considerably, and the swollen foot which has penetrated deeply into the tissue of the fruit-shoot, ar, arche gonia which have remained unfertilized ; p, base of the perianthium ; st, stem (fruit - shoot) ; 6, leaf. (After Hofmeister.) cells of the spores pro- ceed have here the form of a dome and enclose an inner portion which becomes the columella, while the inferior portion of the embryo becomes its foot. In Dendroceros, a foreign form of Anthoceros, it drives tubules resem bling roots into the tissue of the plant on which the sporogonium is seated. There is no want of transitional steps between this method of formation of the sporogonia and that of the Jungermanniese. The species Notothylas is especially noteworthy in this respect. The spores, Avhich proceed from one mother-cell by division into four parts, are enveloped by a membrane consisting of two layers an outer one, which has the character of a cuticle in its great resistance to external agencies, and an inner one, which is composed of cellulose. The plants do not proceed directly from the germina tion of the spores ; there is a pro - embryo of simple structure, which, however, in most cases passes over at its end into the plant itself. In the foliose Jungermanniex the first step in germination is the springing up of a row of cells in the cell at the end of which the growth of the shoot next takes place. The young germ-plant next develops leaves of very simple structure, and at first there generally appear on it only two rows of leaves at the sides. In Jfadula, however, and Frullania, both of them leafy forms, a cake- like cell-surface is first developed in germination ; and it is from one of the cells of the margin of this that the growth of the leafy plant proceeds. CLASSIFICATION* OF LIVERWORTS A. Marchantiacese. I. Riccicas. The vegetative body is a dichotomously branched thallus, on the dorsal side of which are air-chambers, either covered by an epidermis like the Marchanticis, or opening outwards in their whole breadth. On the under side of the thallus is a row of scale-shaped lamellae, which become split up at a later period. The sexual organs are scattered on the dorsal side of the ordinary shoots. In the spore-cavity are neither elaters nor sterile cells II. Corsinieie. The organs of sex are united in groups, which stand in cavities on the dorsal side of the thallus. In the spore- space are sterile cells, which in JBoschia are transformed into elaters. The dorsal side of the thallus possesses stomata. III. Marchantiese. The thallus is ribbon - shaped, and has a layer of air-chambers on the dorsal side opening outwards through a stoma. On the ventral side are two rows of scales. In the spore- space are elaters. The formation of the &quot;inflorescences&quot; in the lowest forms agrees with those of Corsinia in the highest forms they appear as branch-systems. B. Jungermanniaccfe. I. Jungcrmanniex. The vegetative body is a thallus without air-chambers, or is a leafy stem. Between these are transitional forms. The sporogonium is differentiated into capsule, stalk, and foot ; the capsule dehisces by iour valves. (a) Anacrogynous. The apex is not employed in the formation of the archegonium. To this division belong all the thai-