EFFECT OF CLIMATE ON COMPOSITION OF PASTURE PLANTS        367

on the one hand, and rate of growth on the other. We may consider the latter
aspect first.

In tropical and sub-tropical climates the growth of pasture is rapid and
frequently the plant approaches maturity a few months after the onset of the wet
season. In cool temperate or winter rainfall climates growth proceeds much more
slowly and the reproductive stage is reached only after many months of slow
vegetative growth. This might be interpreted as an effect of temperature on
carbon assimilation ; the rate of carbon assimilation, and hence the rate of growth
within certain limits increases with temperature. The amount of carbon assimi-
lated per unit leaf area need not, however, alter, unless there is an alteration in the
ratio of leaf area to total dry weight. The actual facts are as yet undetermined.

Another habitat factor that may affect the growth rate is the length of day.
It is known that short-day types of plants are found in tropical regions and long-
day types in polar and cold temperate regions (13), and Auchter and Harley (14)
have shown that in temperate regions the pasture plants are of a type that form
reproductive organs only during periods of long days. Moreover, Tincker (15) has
shown that the long-day types of plants may be kept in the vegetative stage by
exposing them to short-day conditions. While the length of day is known to have
a complex effect on the growth and development of the plant, we do not know the
precise relationship it bears to carbon assimilation in the pasture plants under
consideration.

We may turn now to the question of mineral uptake. This process is known
to follow a fundamentally different course from that of carbon assimilation. Thus,
Richardson, Trumble, and Shapter (2) have shown that in pasture plants in South
Australia, the rate of absorption of nitrogen and phosphoric acid, i.e., total amount
absorbed in unit time, is much greater in the early vegetative stages than the rate
of synthesis of organic matter; nitrogen and phosphoric acid are utilised in con-
siderable amount during the tillering and root establishment charaterising the
early phase of development. After this early tillering stage, the rate of absorption
falls rapidly.

On the other hand, the rate of carbon assimilation, i.e., the total amount of
carbon assimilation in unit time, is dependent, apart from temperature, light and
carbon dioxide supply, mainly upon the area of green leaf surface and as this
normally increases till the approach of flowering, the rate of carbon assimilation
correspondingly increases and attains a maximum shortly before this stage. Thus
the maximum rate of nitrogen and phosphorus absorption on the one hand, and
carbon assimilation on the other, occur at two different periods of the plant's deve-