486 
WRIGHT : FOLIAR PERIODICITY 
The following list shows the relative deciduous character 
of natural orders containing a varying number of arborescent 
indigenous species :— 
No. of Indigenous 
Natural Order. 
No. of Indigenous 
Arborescent Species. 
Deciduous Species. 
42 
Euphorbiaceæ 
5 
33 
Leguminosæ 
14 
28 
Urticaceæ 
14 
18 
Myrtaceæ 
4 
18 
Rubiaceæ 
3 
13 
Rutaceæ 
1 
11 
Yerbenaceæ 
3 
9 
Lauraceæ 
2 
9 
Meliaceæ 
4 
8 
Tiliaceæ 
1 
8 
Sterculiaceæ 
3 
8 
Sapindaceæ 
2 
6 
Combretaceæ 
4 
6 
Sapotaceæ 
2 
It is a noteworthy fact that the natural order—Euphorbia- 
ceæ —which possesses by far the largest number of arborescent 
indigenous species of any order in Ceylon should be 
characterized by so few trees which become leafless. The 
contention that the latex of these plants is of service during 
periods of drought or in districts which periodically suffer 
from want of rain would perhaps appear tobe supported by 
this fact, as the species concerned are able to pass through 
the hot dry season without becoming leafless. Parkin* 
states that the latex from introduced species flows more 
freely and is thinner in the wet than in the dry season, and 
that in his opinion the laticiferous tubes act as water-stores ; 
when there is plenty of moisture in the soil, more water is 
drawn up by the roots than is needed at once, and finds its 
way into the laticiferous ducts, to be drawn upon when 
required. The indigenous species which are deciduous 
contain very little, if any latex, a condition quite unlike that 
which obtains for the species of Hevea, Manihot, &c., 
* Parkin, J.; Caoutchouc or Indiarubber, Circular of the Royal Botanic 
Gardens, Ceylon, Series 1, Nos. 12, 13, 14, 1899. 
