Ecological Studies on Growth-Form in Bryophytes: II. Experimental Studies on Growth-Form in Mosses

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Abstract
The preceding paper in this series demonstrated correlations between the occurrence or prevalence of certain growth-form types in bryophytes, and variations in habitat conditions. Investigations are now reported on the extent to which growth-form is molded by the environment, or represents an inherited organization of relative rigidity.. In controlled conditions of light intensity and relative humidity, moss shoots from several growth-forms grew erect provided illumination was evenly distributed. With unilateral illumination positive phototropism was displayed, though this factor alone did not produce creeping growth. Low humidity with good illumination and a moist substratum may induce creeping or weakly-ascending growth in rough mats, smooth mats and some dendroid forms; but short turfs and tall turfs with erect branches continue to grow erect. Thus in relation to the direction of shoot growth, some growth-forms appear to be rigidly controlled, others more plastic. Detailed observations on Climacium dendriodes and Thamnium alopecurum, 2 representatives of the dendroid form, are described. This growth-form has a well-defined morphology and relatively narrow ecological amplitude. Field and experimental evidence indicates that alternating stoloniferous and erect growth in Climacium may be associated with seasonal fluctuations in the moisture conditions of the habitat, the initial plagiotropic growth of stolons possibly being determined by the proximity of a moist substratum when the air is relatively dry. Variations of form in Thamnium appear chiefly dependent on the constancy and manner of supply of liquid water. In northeast Scotland these dendroid species are in fact largely restricted to habitats characterized by seasonal fluctuation in their moisture regimes. Some capacity for plastic modification of form in the species concerned appears to be related to the extent of ecological amplitude in each case.

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