The Embryology and Development of Riccia lutescens and Riccia Crystallina

Riccia lutescens and Ricciocarpus natans are forms of the same plant, the former occurring on the ground in summer and autumn when the ponds are dry, and the latter as a floating form. Either form can be changed into the other by altering the supply of water. Therefore, Riccia lutescens should not be regarded as a distinct species. 2. The genus Ricciocarpus has been based largely on characters which do not exist. In my opinion, the only real basis for separating it from Riccia is the more complex structure of the thallus. BISCHOFF did not regard this as a good character for the separation of the genus. 3. The plant is monoecious, antheridia and archegonia being produced in definite groups in the same thallus. The sexual organs appear in autumn when the thalli are growing on the ground and complete their development the following April. Abundance of water is not essential to sexual reproduction, as the plants fruit when kept growing on the soil and supplied with a limited amount of water; therefore the ground form is not sterile, as was the opinion of LINDBERG and GARBER. 4. Plants which have been growing attached to the soil and have been submerged by the filling up of the ponds do not necessarily perish, but are adapted to spend the winter under water and then to break loose by the decay of the older part of the thallus and float upon the water in the spring. 5. The plants are propagated vegetatively by the separation of branches of the thallus, by the decay of the older part, and also by the growth of new plants from cells in the apical region. 6. The sexual organs and fruit of the two species studied agree in their development with the accounts given for the other species of Riccia. There is no rudimentary integument surrounding the archegonium or sporophyte of Riccia natans. The sporogonium of Riccia natans is larger than that of Riccia crystallina and produces a larger number of spores. The only sterile tissue in either is the amphithecium, a single layer of tabular cells surrounding the mass of spore mother-cells. 7. Centrosomes are not present in ceils outside the antheridium nor would I interpret any structure observed in the cells of the sporophyte or the spore mother-cells as a centrosphere. 8. Bodies which resemble centrosomes, and which are considered to be true centrosomes by certain authors, occur in the cells of the antheridium. These bodies do not have genetic continuity, but arise de novo with each division. They do not disappear after the last division of antheridial cells but remain in the spermatids and later become blepharoplasts. 9. In the earlier divisions of cells in the antheridium, the spindle is arranged parallel to the long axis of the cell, but in the last division,the spindle is placed diagonally in the cell. No wall is formed between the two cells produced by this division, each of which becomes a spermatozoid. Thus two sperms are produced from each cuboidal cell. 10. In the developing sperm, the blepharoplast takes a position on the membrane of the cell and the two cilia grow from it, the nucleus becomes almost homogeneous in structure and crescent-shaped, almost enclosing the cytoplasm. The mature sperm consists of the nucleus, the cytoplasm, and cilia which have received material for their growth from the blepharoplast and probably also from the material surrounding it. 11. The amount of chromatin in the nucleus is small. There is no nucleolus present unless the masses of chromatin which are found in nuclei which are undergoing repeated division be interpreted as nucleoli. 12. The number of chromosomes is four for the gametophyte and eight for the sporophyte. 13. The cytoplasm of the spore mother-cells appears to be a fine reticulum, in which are numerous granules usually located at the point of intersection of the fibres of the reticulum. 14. The mature spore contains a large quantity of oil together with a small amount of granular matter. The nucleus of the spore is very small.