

1920] HAUPT—FOSSOMBRONIA 323 



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Growth of the main axis and branches is by means of a dolabrate 

 (zweischneidig) apical cell (figs. 4, 5), with which are associated 

 simple ventral mucilage hairs (figs. 7, 22) which may be several 

 cells in length. Cavers (2) states that each lateral segment of the 

 apical cell of Fossombronia, by 2 transverse divisions, forms 3 

 horizontal cells, the upper and lower cells developing the stem and 

 the middle cell forming a leaf, according to the same method as 

 occurs in Blasia. 



The leaves are borne in 2 dorsal rows; they are more or less 



erect, obliquely inserted on the stem, closely imbricate, and pale 



green (fig. 2). The ventral surface of the thallus is entirely devoid 



of leaves. Hill (5) notes that the leaves become paler and whitish 



with age. The shape of the leaves varies from somewhat quadrate 



to slightly obovate; they are very crisped and have subentire 



margins which occasionally bear a few feeble crenulations at the 

 apex. 



The cells of the stem and leaves contain numerous small pe- 

 ripheral chkjroplasts. Considering the small size of the plant, the 

 cells are relatively large. Mitotic divisions were very rare in the 

 material studied ; the best mitosis seen was that of a late metaphase 

 in the apical cell (fig. 3). From a study of this figure it was 

 estimated that the haploid number of chromosomes is 4, although 

 this fact cannot be stated with absolute certainty, as no other 

 stages of mitosis equally favorable for chromosome counting were 

 found. 



There can be no doubt that the 2 rows of lateral outgrowths 

 from the axis of Fossombronia represent true leaves. The develop- 

 ment of such a plant body from a form like Pallavicinia Lyellii, 

 which consists of a midrib with thin, one-layered lateral wings 

 slightly undulate on the margins, is very logical. Symphyogyna 

 aspera might be taken to illustrate a second evolutionary stage, 

 as in this plant the wing margins of the thallus are distinctly lobed. 

 Among the Codoniaceae, Blasia represents a still farther advance, 

 as in this case the lobes are even more distinct and regular, and 

 the step from this condition to that of Fossombronia is perfectly 

 natural. The plant body of Noteroclada is still more distinctly 

 leafy, and in Treubia the axis bears 3 rows of leaves formed by an 



