480 



BOTANY 



PART II 



summit of the stem. This tissue along with the layer derived from the venter of 

 the archegonium forms an investment that is later broken through by the growing 

 embryo. The upper portion derived from the archegonial wall is carried up as 

 the calyptra, while the lower portion forms a sheath round the base of the 

 sporogonial stalk. The origin of the calyptra is similar in many Liverworts (e.g. 

 in the Marchantiales) ; in others, however, the base of the embryo grows more or less 

 deeply into the tissue of the thallus or stem below the archegonium. In special 

 cases the tissue adjoining the archegonia forms a pouch-like structure (marsupium) 

 enclosing the archegouium and embryo ; this often grows down into the soil and 

 represents a peculiar organ of protection and nutrition. 



The development of the sporo- 

 gonium exhibits a remarkable 

 variety. In the lower Liverworts 

 (Marchantiales) the zygote divides 

 by transverse and longitudinal 

 walls into 8, then by further radial 

 walls into 16 cells, following on 

 which comes division into external 

 and internal cells by periclinal 

 walls (Fig. 439). The foot and 

 short stalk of the sporogonium 

 come from the lower half of the 

 embryo and the capsule from the 

 .upper half, the internal cells of 

 which form the archesporium and 

 give rise to the sporogenous tissue. 

 The cells of this become in part 

 spore -mother -cells, while others 

 FIG. 439. Development of the Sporogonium of Corsinia remain sterile and serve at first 

 marchantioides, one of the Marchantiaceae. A, The ag nutritive cells to the developing 

 zygote divided into 16 cells B, The lower half of the ^ ^$ C). ^ th 



embryo developing as foot, the upper as capsule ; w, 



wall cells; ar, archesporium (x 170). C, Older sterile cells usually grow into 

 sporogonium. The archesporium has given rise to spindle-shaped structures with a 

 spore-mother-cells and small sterile cells which in spiral thickening of the wall 

 Corsinia do not develop further into elaters. (x 90.) (elaters) these on the open i n g o f 

 (After K. MEYER.) 



the capsule assist in the dispersion 



of the spores. Only in the Ricciaceae do all the internal cells become spore-mother- 

 cells, the whole sporogonium being simplified to a spherical, unstalked capsule with 

 a wall of one layer of cells. 



In the higher Liverworts (Jungermanniales) the zygote first undergoes a number 

 of transverse divisions ; the lowest cell becomes sometimes after a few divisions an 

 absorbent organ while the upper cells give rise to foot, stalk, and capsule. In 

 addition to the spores, sterile cells, which usually develop into elaters, are formed 

 from the sporogenous tissue. 



The Anthocerotales are Liverworts which deviate considerably as regards the 

 construction of the capsule from those described above and in some respects 

 approach the Mosses (cf. p. 483). 



In the Mosses the sporogonium has a columella which is an axile strand of sterile 

 tissue serving for the conduction of materials ; around this the archesporium is 

 arranged as a, usually single, layer of cells. In the Sphagnales (Fig. 452 C) and 

 the Andreaeales the archesporium extends as a dome over the summit of the 



