262 THE STRUCTURE OF FLOWERS. 



tensis. Fig. 60,* a, represents a corolla, whicL. is very small, 

 but open ; b represents the two fertile stamens ; the anther- 

 lobes instead of being 

 horizontal are erect, and 

 face each other. The 

 , u stigmas curl back be- 



Fig. eO.-Salviaclandestina: a, corollas ■,b,s.nthers; ^^^een them, aud are rc- 

 c, style and stigmas. markablj long, C. 



The Origin of Cleistogamy. — We are now in a position 

 to trace the causes of cleistogamy. Cleistogamous flowers 

 nearly always occur on plants otherwise, or at least their 

 allied species are, adapted for intercrossing, and include four 

 genera of anemophilous plants. The first cause or influence 

 is the arrest of the reproductive energy in the conspicuous 

 flowers, which often set no seed at all. 



Whatever the primary cause of that may be, a very 

 common result in perennials is to increase the power of 

 vegetative methods of multiplication, as in the case of many 

 bulbous and tuberous plants. 



This, however, is not a special feature of the plants which 

 bear cleistogamous flowers. It would seem, therefore, that 

 the reproductive energy being checked in one form of flower, 

 it, so to say, breaks out in another. But there are several 

 influences at work, and a very obvious one is temperature ; 

 for the same species may behave very differently in one 

 country with a high mean annual temperature, from what it 

 does in another with a lower one. Thus, Viola odorata does 

 not produce cleistogamous flowers in one part of Liguria, 

 where the conspicuous flowers are perfectly fertile ; while 

 they are mostly barren in England. On the other hand, 

 cleistogamous flowers are produced by Violets near Turin, 



* From a specimen growing at Kew. It is cleistogamous at Halle 

 (see below, p. 263). 



