Studies of the growth of moulds.―I

It is a remarkable fact that in the whole range of botanical literature there are so few references to the influence of humidity on the rate of plant growth. And this not because tire significance of humidity has passed unnoticed, for those botanists who during the latter half of the past century were tracing the history of the passage of plants from an aquatic to a sub-aerial environment were fully aware that the loss of water from the plant to the atmosphere was a factor whose influence could not be neglected. Both in their writing and investigation there is a constant attempt to explain how, by an increasing adaptation of plant structure and methods of reproduction, the difficulties of the loss of water to the atmosphere have been progressively overcome and growth in habits with a minimum of water has been finally attained. Experimental methods have been Used in the study of transpiration and much is known of its relation to relative humidity and other external factor. But even here much of the work has been primarily an attempt to prove how admirally leaves are adapted to withstand excessive water losses and how the so-called xerophytic characters still further reduce these losses. It is only quite recently that this bias has been overcome and that direct measurements of actual water Iosses have been made, that the rate of loss has been correlated with some of the many external factors influencing the process, and that a purely physical explanation has been advanced and not one based on the presupposed needs of the plant.