CLEISTOGAMOUS FLOWERS 397 



germinating in the anthers and the pollen tubes piercing 

 their walls. The cleistogamous flower, in this case, and 

 in other cases where reduction is more marked, corresponds 

 in the main to an early stage in the development of the open 

 flower ; at this stage, however, functional maturity of the 

 germ cells is reached. The most general feature is the 

 reduction of the corolla, which is completely absent, for 

 example, in Cardamine chenopodiifolia. In normal develop- 

 ment the corolla generally appears after the other floral 

 organs. 



The conditions in which cleistogamous flowers are 

 produced favour this interpretation. They tend to be 

 formed either when growth of the plant is predominantly 

 vegetative, or under conditions of malnutrition. Thus, 

 frequently they appear early, preceding the open flowers, 

 as in Impatiem noli-me-tangere. A striking example of this 

 is the Brazilian Cardamine chenopodiifolia, which bears 

 cleistogamous, subterranean flowers on plants which have 

 only developed a few pairs of leaves (Fig. 61). The touch- 

 me-not may be forced to produce cleistogamous flowers by 

 growing it in starvation conditions on dry soil. Many 

 plants, such as the pea or the shepherd's purse, form flowers 

 which never open at the end of their flowering period when 

 the supply of food substances is falling low. 



There are all gradations between plants bearing only 

 normal flowers, some of which may not open in unfavourable 

 conditions, and those which produce only cleistogamous 

 flowers, e.g. Salvia cleistogama. Our native typically 

 cleistogamous plants are all of the intermediate types — Viola 

 odorata, Jiinciis biiffonins, Lamium amplexicaule, Stellaria 

 media, Oxalis acetosella. Not very far removed from 

 cleistogamy, so far as eff'ect is concerned, are those plants in 

 which autogamy takes place as the flower opens, as in the 

 oat, the barley, and the wheat. Although the formation 

 of cleistogamous flowers is evidently controlled to an 

 important extent by external conditions, we must neverthe- 

 less assume the presence of an inherited tendency to their 

 production. 



