Some Effects of Humidity on Sexual and Asexual Reproduction in Phascum Cuspidatum Hedw.

Abstract

An apparatus has been designed and constructed, to supply streams of air to mosses growing under controlled conditions. Each air stream could be adjusted to a known humidity, and the humidities could be measured continuously. The relevant literature has been reviewed in two parts. The first part includes an account of atmospheric humidity together with the various hygrometers now available, and the second part contains an account of the water relations of the bryophytes. Details of the design and construction of the apparatus are set out, together with notes on its use and a discussion of its performance. Two quite separate aspects of the effects of humidity have been investigated. Haploid sexual plants have been used to determine the effect of humidity on antheridial dehiscence, and diploid apogamous plants, to investigate the effects of humidity on their leafless axes. Past experience has shown that it is difficult to initiate the sporophyte generation in Phascum_cuspidatum growing in pure culture. This has been referred to the failure of antheridial dehiscence and two explanations of this suggest themselves. The conditions in culture vessels are very unlike field conditions in two respects, the humidity is always at or near 100% r.h., and the air is still. Both of these explanations have been investigated and it is shown that the necessary stimulus is mechanical rather than a change in humidity, Conclusions, and a discussion of these findings are presented before the account of the other investigation.  It has been suggested by several investigators that the development of the leafless axes of the diploid apogamous plants can be controlled by the "dryness" of the culture medium, and that these structures may function as propagules if sporangia do not form. The investigations showed that the plants are extremely sensitive to desiccation. From this, and other considerations, it has been possible to deny both of these earlier suggestions.