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The Morphology of Riella affinis. I. Germination of the Spore and Development of the Thallus

R. H. Thompson
American Journal of Botany
Vol. 28, No. 10 (Dec., 1941), pp. 845-855
Stable URL: http://www.jstor.org/stable/2436862
Page Count: 11
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The Morphology of Riella affinis. I. Germination of the Spore and Development of the Thallus
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Abstract

The vegetative growth of Riella affinis is marked by three distinct phases; the sporeling phase in which growth is initiated by a pair of apical cells; the juvenile thallus phase in which growth is initiated by an intercalary meristem; and, the adult phase in which growth is initiated by a single apical cell. In the sporeling phase the first division of the germ tube produces a single apical cell with one cutting face. The apical cell divides once transversely and then divides longitudinally to produce a pair of apical cells. These successively cut off four pairs of derivatives and then metamorphose into oil cells. Successive division in two planes of the derivatives produces a typical spatulate sporeling thallus in which cell lineage from each derivative is distinct and a definite pattern of growth is exhibited. The basal cells arising from the first pair of derivatives cut off by the pair of apical cells remain embryonic and function as the intercalary meristem of the juvenile thallus. The development just described for R. affinis differs greatly from that described for other species of Riella. The fact that the pattern of sporeling growth in R. affinis can be applied accurately to published figures of similar stages in other species of Riella suggests very strongly that early development in other species of Riella is the same as that in R. affinis. Division in the intercalary meristem of the juvenile thallus is predominantly transverse. Division of the derivatives from the upper region of the meristem is in two planes and produces the green ovate part of the juvenile thallus. Division of the derivatives from the lower region of the meristem is transverse and produces the colorless lanceolate part of the juvenile thallus. Rhizoids are produced by the cells of this part. When the juvenile thallus reaches a length of five or six millimeters, meristematic activity becomes limited to one or both margins of the intercalary meristem. In this limited meristematic region a single apical cell becomes established which then initiates all growth of the adult thallus. During the period of transition and establishment of a single apical cell the ovate part of the juvenile thallus is pushed to one side. The apical cell has two cutting faces, ventral and dorsal. Ventral derivatives of the apical cell divide in three planes to produce the round, multicellular rib. Dorsal derivatives of the apical cell divide in two planes to produce the unistratose wing. In R. affinis growth of both ventral and lateral scales is initiated by an apical cell which cuts off three derivatives and then matures into a terminal mucilage cell. The body of the scale is produced, in each case, by further division of the three derivatives.

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