Bryophytes: The thallus

In many cases it will not be necessary to make a special preparation for the study of the thallus, since preparations of antheridia, archegonia, or sporophytes may include good sections of vegetative portions. This is particularly true of forms like Riccia, where the various organs are not raised above the thallus. In forms like Marchantia, where the antheridia, archegonia, and sporophytes are borne upon stalked receptacles, it is better to make separate preparations to show the structure of the mature thallus. Sections intended to show the structure of the mature thallus should be 15 to 25( in thickness, but sections to show the growing point and development of the thallus should not be thicker than 10(. The apical region of the Junger- manniaceae (Figs. 65, 66) affords an excellent opportunity for studying the development of the plant body from a single apical cell. If mixtures containing osmic acid are used for fixing, there may be difficulty in the staining, even after using peroxide of hydrogen.

Chromo-acetic mixtures, without osmic acid, are better for the apical region. Chromo-acetic acid, followed by Delafield's haematoxylin, is good for the apical cells and developing regions, but a light counter-stain with erythrosin improves preparations of the mature thallus. Safranin, with light green, or safranin. with crystal- violet, will give clear views of the growing region. The latter combination is particularly good for forms like Pellia, where even the apical cell is more or less vacuolated, since it not only brings out the cell walls, but stains plastids and other cell contents (Fig. 66). The chloroplasts and leucoplasts are well differentiated by this stain. After corrosive sublimate-acetic, a vigorous staining in a mixture of acid fuchsin and iodine green often brings out the walls very sharply. After corrosive sublimate-acetic the material may be stained in bulk with alum cochineal or alum carmine, thus giving fairly good preparations and saving considerable labor.

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