STEM OF MNIUM. 



229 



of the stem is prolonged outwards into a unilamellar plate of cells, 

 which represents the leaf-wing (/) running outwards from the 

 stem. Cross-sections, 

 which are taken from 

 the lower, leafless, 

 strongly- browned part 

 of the stem, show the 

 walls of the peripheral 

 layers of cells coloured 

 dark-brow r n. From in- 

 dividual cells of the 

 surface have grown 

 long, brown - walled, 

 repeatedly - branched 

 threads of cells, which 

 are designated hair- 

 roots or rhizoids 

 (r). These rhizoids 

 have oblique partition- 

 walls. Under many 

 of such partition 

 walls, and always 

 under their side most 



FIG. 90. Cross-section through stem of Minium un- 

 ulatum. t, conducting bundle ; c, cortex ; e, epidermis ; 

 leaf-wing ; /-, /-, rhizoids ( x 90). 



remote from the base (see below r, left hand) arise branches, which 

 branch again and again in the same way. Only the growing apices 

 of the rhizoids have colourless walls. 



The closest similarity with such rhizoids, in respect of branch- 

 ing and the oblique position of the dividing walls, is shown by 

 the proembryo of the typical leaf-bearing mosses, the so-called 

 protonema, which is developed from the germinating spore. Its 

 branches, however, so far as they do not penetrate into the soil, 

 are colourless, and contain numerous chlorophyll grains. The 

 buds, which develop into moss-stems, are side-branches of this 

 protonema (compare Fig. 91). The close relationship of rhizoids 

 and protonema is shown also in the fact, that rhizoids, if kept 

 damp and exposed to light, can develop a protonema which can 

 give rise to numerous new r plantlets. It needs only to lay a turf 

 of Mnium with the under side upwards, and to keep it damp, in 

 order to produce numerous green protonema-threads from the 

 rhizoids. The protonema as seen with the naked eye reminds us 

 of tufts of terrestrial Vaucheria. 



