32 THE FLORA OF SINGAPORE. 
is not rare however to find certain plants of Pigeon orchid which 
do not flower on the regular day, but have a distinct day which 
they appear to keep to with equal regularity. A curious fact is 
that another species of Dendrobium (D. eriniferum) invariably 
flowers in Singapore on the day preceding that of D. crumenatum, 
whenever that happens to be. It might be thought that the 
weather in the district in which the plant was growing was the 
influencing agent, but this appears to have but little effect on the 
orchids. On one occasion (Dec. 5, 1893) the pigeon orchids 
developed their flowers so far that they were obviously ready to 
open them on that day, but an extraordinarily heavy rain retarded 
them, and the flowers opened the next day, but except in cases like 
this the weather previous to the flowering does not seem to make 
any dilference to the date of flowering. Some few plants have 
a regular annual flowering season, such are Calanthe cureulr- 
goides in September, and Grammatophyllum, July and August. 
A certain number of trees flower only every fifth year, notably 
the Dipterocarper, Every fifth year there appears to be an 
average higher temperature, and @ period of greater dryness in 
May or June than in other years, and then and only then is it 
possible to obtain flowers of these trees. Such trees are most of 
the Dipterocarpen, vanthophyllum Kunstleri, Careya sp. In this 
case also all the plants in a given district flower simultaneously. 
It can easily be understood that it is very important to a plant 
that all should flower on the same day in order that they may be 
cross-fertilized by the insects that visit them, and this is especially 
the case in plants in which the flowers last but a single day, as 
in the case of the Pigeon orchid, but it is difficult to see how 
this is brought about. A good many trees seem to flower even 
less often than this and tkere are not a few which though 
apparently full grown healthy trees have not given flowers once 
in ten years. 
Colors of flowers. 
As is well kuown the colors of Htowers depend to a large 
extent on the kind of insect fertilizer for which they are destined 
to prove attractive. The most abundant insects in the thick 
jungle are the flies (Diptera) and these appear often in encrmous 
abundance when certain trees are in bloom. The Oaks aad 
