238 FLOWERS AND THEIR WORK. 



dichogamous, but not completely so, there being usually a 

 short period during which self-pollination becomes possible. 

 To effect this there are sometimes special contrivances, such 

 as the curling back of the stigmas to reach the pollen, e.g. 

 Love-in-a-Mist (Nigella), Composites, Campanulaceae. 



A very special adaptation for self-pollination is the pro- 

 duction of cleistogamic flowers. These are closed flowers 

 produced late in the year by certain plants which had 

 previously produced ordinary flowers e.g. Sweet Violet, 

 Wood Sorrel, Henbit Dead-nettle (Lamium amplexicaule) . 

 They are small and inconspicuous ; the calyx never opens, 

 and the stamens and pistil are developed in a closed case. 



In our study of flower-mechanisms we are too apt to forget 



(1) that self-pollination occurs regularly in flowers which are 

 neither unisexual, completely dichogamous, nor self-sterile, 



(2) that it is rarely much inferior to cross-pollination in its 

 results, and (3) that it is always better than no pollination. 



In Violets the flowering-period of the ordinary insect- 

 pollinated flowers is short (rarely more than six weeks) 

 and the number of flowers produced is small especially 

 in Sweet Violets, in which flowering only lasts for a month 

 and a plant produces only about six flowers, each flower 

 lasting about a week in a condition ready for pollination. 

 Except in very bad weather these spring flowers are visited 

 and pollinated by bees, but flowering does not stop when no 

 more of the ordinary flowers are to be seen. The plant goes on 

 producing flower-buds until autumn. Those which succeed the 

 early (normal) flowers do not open but are self-pollinated, so 

 that fruits are continually formed during summer. Finally 

 in autumn a few buds are laid down for the normal insect- 

 pollinated flowers which will open next spring. 



In Sweet Violet these closed self-pollinating (cleistogamic} 

 flowers have five very small petals and five stamens, but in 

 Dog Violet there are only two stamens (the lowest ones). 

 The anthers produce few pollen-grains (why few ?) and do 

 not open; the grains sprout inside the anther, and the 

 pollen-tubes grow through the anther-wall and the style to 

 reach the ovules. The formation of these flowers is partly 

 dependent on shade; they are always shaded by the leaves 

 of the plant itself. If a plant is kept in feeble light, it will 

 usually produce only these cleistogamic flowers. 



