of blue-green algae. The algae commonly found in these situations, are colonies of Nostoc embedded in soft mucilage, colonies of another blue-green with tough mucilage and filaments of Scytonema. Under the microscope, these appear as masses of algal cells or filaments. The rhizoids of the young plants enter into the algal masses and come into intimate contact with them. In a few more days, the young plants produce a great number of gemmae from the axils of the leaves near the tip of the stem. These get detached and are distributed beyond the area of the wall originally covered by mosses, and get mixed up with the algae growing in these regions. In the outskirts of the older patches, the rhizoids of the young plants grow into the new substratum and give rise to protonema filaments which are covered by masses of blue green algae. Gradually new moss plants appear in these regions and the spreading of the moss on the wall continues. It is a matter of constant observation that no new plants develop in regions of the wall where the algae have not already appeared.
Protonemal Stage.—A microscopic examination of the outskirts of the moss patches, where to all outward appearance no moss plants but only algae are found, show masses of protenema filaments or germinating gemmae. The protenema filaments arise from rhizoids of young plants further up or from hibernating rhizoids of the previous season which had extended so far. The protonemal filaments branch frequently and are mixed up with algal masses and soil particles.
In the preparation of slides, the lime substratum was dissolved with dilute HCl and the sand particles left behind were removed by careful teasing with fine pointed needles. The material for mounting, both in the case of protonema and also in the case of rhizoids of the moss plants in different stages, was prepared in this way, stained and mounted in glycerine.
The greater part of the protonema branches were bright green in colour with prominent chloroplasts. But the tips of the filaments in many cases and some of the branches gradually became colourless and finely branched. (Fig. 2.) This colourless portion was always in intimate contact with the algal masses. During teasing, it was often noticed that the green portion was easily detached from the algal masses but they always had their ends broken. Light crushing of the algal mass and differential staining showed that the colourless tips of the protonemal filaments extended into the gelatinous masses of algae and there spread in between the algal cells. In some, where the algae form soft gelatinous masses, the branches had short beaded cells which occupied the centre of the mass. In others, where the algae had tough
