160 



MONOCOTYLEDONS 



plants growing on dry soil and during the dry season. The 

 leaves are dark-green, which is likewise referable to the habitat 



of the plant. Dark-coloured things, 

 we know, absorb more heat than 

 light-coloured ones. The higher the 

 temperature of the leaves of a plant 

 rises, the more abundant is the 

 evaporation and hence the growth 

 (see II. Part, Transpiration). The 

 damp surroundings, which naturally 

 check the evaporation, are thus com- 

 pensated for by the dark colour of 

 the leaves. 



The thick, fibrous roots which 

 spring from the stem immediately 

 over the tubers, are few and small; 

 they are sufficient for the supply of 

 water from the soil which is constant- 

 ly humid during the time of its 

 growth. 



3. A slender, leafless stem, called 

 scape, about six inches high, rises 

 from the radical leaves and lifts the 

 few Flowers over the tips of the grass 

 around it. They thus become visible 

 to passing insects which have to 

 fertilize them. 



Each flower is attached to what 

 f>eems to be a short twisted flower-stalk rising from the scape 

 in the axil of a conspicuous bract, but it is really the ovary, 

 which can be proved by cutting it through and thus laying the 

 ovules inside bare. 



The perianth is composed of two sets of three leaves each. 

 The middle petal of the outer set and the two upper or lateral 

 petals of the inner set are bent together and form a hood to 

 protect the inner organs. The two other outer petals are ex- 

 panded, and the middle ])etal of the inner set, generally called 



Fig. 147- — Flower spike of 



Habenaria rolundifolia 



(2 times magnified). 



