262 GENERAL BOTANY 



Conversely there are many cases of structures 

 which are homologous but have different functions, 

 and differ in physiological value. 



4. Before we take leave of these winged and hairy 

 fruits there is an interesting and suggestive fact which 

 we may notice about them. It is that all these fruits 

 enclose but one seed only. 



In the COMPOSITE and Valerians the ovary though 

 theoretically formed of two carpels (the origin of the 

 bifid style) has but one cell and but one ovule in it. 

 In the DIPTEROCARPE.E, as SHOREA (fig. 63) and 

 HOPEA, and in HIPTAGE (fig. 61), the ovary is three- 

 celled, and, in the case of the DIPTEROCARPE^E, each 

 has at first two or more ovules. But as the ovary 

 ripens into the fruit, one only of the cells is developed, 

 and in it one only of the ovules becomes a seed. 

 Even with PONGAMIA, PTEROLOBIUM, and PTERO- 

 CARPUS which belong to a family the fruit of prac- 

 tically all the other members of which is a legume 

 containing many seeds, there is but one in the fruit. 



The reason for this is connected with the fact that 

 the supporting efficiency of a wing or tuft of hairs, 

 depends on the relation between the resistance it 

 offers to the air and the weight of the whole struc- 

 ture. So that obviously a fruit with one seed only 

 in it will be carried further, because it is lighter, 

 than one with several seeds. 



Most of these one-seeded fruits, too, are indehiscent r 

 and since the fruit wall itself (ovary) does not open, 

 but remains protecting the seed, the testa may be 

 very thin and so the material necessary for making 

 it, is saved. 



