588 Thoday . — Behaviour during Drought of the Leaves of two 
In July, 1920, I collected small twigs of this plant on the Matoppo Hills, 
near Bulawayo, in the dry folded condition, enclosed them in a corked 
tube, and on my return to Cape Town a fortnight later found the water 
content to be only about 7 per cent. Such plants as this must be completely 
air-dry and dormant. 
Passerina , on the other hand, still transpires slowly and is not dormant, 
however sluggish its vitality may become under such extreme conditions. 
Determinations of the water content of other plants show that, although 
the minimum is so far lowest in the case of Passerina , the shoots of not 
a few sclerophyllous plants may have a water content falling considerably 
below 50 per cent, in the 
summer. 
As might be expected, 
a higher percentage of 
water is found at other 
seasons of the year. Shoots 
of Passerina collected in 
mid -September, 1920, had 
a water content of 59 -61 
per cent. Even this figure 
is lower than might have 
been anticipated in shoots 
which were collected after 
rainy weather, towards the 
end of a wet winter, early 
on a cloudy morning. After the first week of summer weather, with a 
fairly strong dry south-east wind and brilliant sunshine, the water content 
of shoots of both species, collected in the afternoon, was 56 per cent. 
Mechanism of Closure . 
When a transverse section of a closed leaf is put into water, it expands 
at once and the groove opens widely. In order to estimate the degree of 
expansion involved, adjacent sections cut dry were mounted, one in oil, the 
other in water, and camera lucida outlines compared. 
Cases were often observed of thin sections, either dry or mounted in 
oil, having their edges overlapping, showing that in the intact leaf the edges 
were pressed together under tension (Fig. 1). 
On comparing the sections the change of dimensions proved very 
considerable. It was most apparent in the mesophyll, where in the closed 
condition the palisade tissue was so condensed that the individual cells 
were often difficult to distinguish, while the spongy tissue was relatively 
still more contracted in volume. The cells of the outer epidermis were also 
contracted in height. 
1 ■ ■ i—i 1 i < — 1—1 — 1 
tenth mms. 
Fig. i. Tracings of adjacent sections from a leaf of 
Passerina cf. falcifolia in which the groove was tightly closed ; 
left, in oil, the edges overlapping ; right, in water, groove 
open, tissues expanded. 
