AUTOGAMY IN CLEISTOGAMIC FLOWERS. 395 



year but one after their being sown, in addition to cleistogamous flowers, beautiful 

 scented blossoms of a violet colour which were borne on erect stalks and in due time 

 unfolded their petals. 



This result throws some light on the nature of the stimulus which causes the 

 formation of the flowers in question. No open, aerial flowers were produced by 

 Viola s&pincola so long as it grew in the cool shade of a dense wood, but when 

 transferred to open ground, accessible to sunlight, such flowers were developed. 

 One can hardly err in ascribing to the sun's rays a very important influence in 

 stimulating plants to the inception of flowering shoots, especially such as bear 

 blossoms possessing bright-coloured petals. Indirectly, however, this advantage 

 accrues to the plants in question that, living as they do in the deep shade, where no 

 bees would, in any case, visit them, even if they had open flowers, they can confine 

 their constructive energy to the inception and development of cleistogamous flowers 

 and save themselves the trouble of producing open flowers adapted to cross-pollina- 

 tion (but useless in the place in question). If the spot where the Violet grows becomes 

 exposed to the sunlight through the trees shading it being blown down or felled, 

 humble- and hive-bees make their appearance in search of honey, and, buzzing from 

 flower to flower, cross one with another. In such circumstances the open, sweet- 

 scented Violet blossoms are in request, and the same plant-individual, which for 

 years in the dark shade has developed none but cleistogamous flowers, is now stimu- 

 lated by the sun's rays into producing flowers with expanded petals. 



A similar instance is aflbrded by the Henbit Dead-nettle (Lamium amplexi- 

 caule), which grows on cultivated ground — in kitchen-gardens, vineyards, and 

 amongst crops. This plant bears two kinds of flowers, viz. some with purple 

 corollas 15 mm. in length, which keep the entrance leading to their honey wide 

 open, and, secondly, cleistogamous flowers with abortive corollas and small green 

 calices, which remain closed. As is the case with many other annual weeds, Dead- 

 nettle plants which have germinated late in the season maintain their vitality 

 through the winter and into the following spring, and accordingly they may be 

 seen at all seasons flourishing, fresh and green, in situations such as are mentioned 

 above. Flowers, too, are initiated and developed by them at all seasons of the year, 

 but it is interesting to note that only in the warm summer, when flower-seeking 

 msects are about, are the beautiful purple corollas of this plant to be seen; in the 

 late autumn and early spring, when it is cool, and there are no flower-seeking 

 msects, this Dead-nettle is able to do without the luxury of corollas, which are the 

 means of alluring insects, and as a fact only cleistogamous flowers make their 

 appearance at those seasons. It must not, of course, be imagined that the plant 

 exercises an intelligent discretion of its own when it abandons the development of 

 corollas. The connection between this effect and the aforesaid conditions is indirect, 

 and we must conceive that the nature of the stimulus which results in the inception 

 of flower-buds is diflferent, when a plant is subject to the influence of the short days 

 and low temperature of late autumn and early spring, from what it is under the 

 conditions prevailing on warm summer days. 



