Summary.11. Under ordinary environmental conditions of temperature and sunlight the growth of peas, as of barley, is seriously hindered by overcrowding, even when each plant receives a similar supply of food and water. Not only is less dry weight produced, but the pods become thin and distorted and fail to develop their seeds properly.2Growth tends to be depressed in hot sunny weather when no protection is afforded. The chief detrimental factors concerned appear to be high temperatures at the roots associated with strong and prolonged sunshine, though the two factors acting individually are much less potent for harm. Under these conditions crowding shelters the roots from overheating and the leaves from too much sunlight, and up to a certain point crowded plants make better growth than those spaced well apart. Overcrowding, however, still depresses growth, probably because the light and root temperature reductions are too great.3Provided insolation is not excessive the amount of daily fluctuation of root temperature over a total range of about 22oC. (6–7‐28‐9oC.) has comparatively little influence upon growth; high maxima and low minima give similar results to low maxima and relatively high minima, provided the average mean temperatures are not too dissimilar.4With high root temperatures a difference in the degree of insolation or in the angle of incidence of the sun's rays may have a considerable influence on growth, a slight easing off of the solar conditions enabling much better growth to be made.5With very strong sunshine reduction of high maximum root temperatures (from 29oC. upwards) allows of satisfactory growth, when unprotected plants are rapidly killed. The inhibitory action of too high temperatures at the roots is thus clearly shown.Nevertheless, the growth so made is less good than under more normal conditions of insolation, thus demonstrating the harmful action of too powerful sunlight, when all the root temperatures rule high.6Boot temperatures appear to be of greater importance than atmospheric temperatures, as good growth can be made in hot atmospheres provided the roots are kept relatively cool.7There is some reason to believe that the minima are of as much importance as the maxima,i.e.that plants can withstand very high maximum temperatures provided there is a considerable drop to the minima, but cannot put up with the constant conditions of heat induced by fairly high maxima, and high