54 



INTRODUCTION 



with the stigmas by their extremities, and the pollen-grains from which they spring 

 are drawn out of the anthers. 



As previously shown by the researches of Dan. MuUer, there are a few slight 

 modifications of the process of fertilization in various species of Viola, for here the 

 pollen-grains do not remain enclosed in the anthers under all conditions. 



I should like to add to these descriptions of H, v. Mohl that the cleistogamous 

 flowers of species of Drosera are very well adapted for observations on the phenomena 

 that have just been considered. 



In D. rotundifolia (at Kiel) open flowers are extremely rare ; as a rule, only 

 buds, bud-like flowers, and fully formed fruits are found on an inflorescence. The 

 youngest buds of about 1^2 mm. in length are best suited for orientation. In 

 these the moderately coarse green sepals and also the very delicate white petals are 

 readily detached, exposing the ovary with its three or five short, curved styles, which 

 bear terminal stigmas in the form of small swellings. The stamens are closely 



applied to the ovary, and are of such a length that 

 the pale two-chambered anthers occur at about three- 

 fourths of its height. (Cf. Fig. 7.) 



In flowers that are further developed it is more 

 difficult to obtain a knowledge of the relations, owing 

 ^, . _, to the fact that the pollen-grains have produced their 



Fig. 7. Cleistogamous Flower . . 



of Drosera rotundifolia z., before tubcs in the form of fine white threads which have become 



the formation of pollen-tubes. r y ,^ ^ ^ ^i i i i 



Sepals and petals have been SO firmly attached, uot Only to the Stigmas but also to 

 removed. X 12 approximately, j^g petals, that when an attempt is made to remove 



(From nature.) '^ ' ^ 



the floral envelopes, the anthers are torn out of their 

 place. Owing to this I have not succeeded in opening flowers in which fertilization 

 was going on, without disturbing the position of the essential organs, and tearing 

 the filaments from their insertions. 



When the flowers reach a length of 3 mm. they are already fertilized, and the 

 white petals, which are united to the withered anthers by pollen-tubes, begin to grow 

 out between the previously closed tips of the sepals, owing to the increased size of the 

 carpels, much as in Oxalis and Impatiens. The corolla, previously enclosed by the 

 calyx and which thus becomes externally visible in the form of a whitish point, next 

 withers, so as to look like a brown speck on the top of the ovary, which goes on 

 elongating as the seeds mature, and continues to be surrounded by the persistently 

 growing calyx ^ 



^ The explanation of the predominance of cleistogamy in the sundew may be found in the 

 circumstance that the small flying insects which could effect cross-fertilization are attracted by 

 the glistening drops on the numerous glandular hairs of the leaves, to such an extent that they make 

 no attempt at cross-pollination, but fly to the hairs. Owing to the continual capture of insect-prey 

 open flowers are useless for the sundew, and it therefore develops cleistogamous ones. 



In the case of our other plants with cleistogamous flowers the conditions are different : 

 in Oxalis the cleistogamous flowers develop only in June and July, at a time when numerous other 

 flowers attract the cross-pollinating insects, which would pay little attention at that time to the 

 concealed and quite inconspicuous flowers of this plant. In spring, on the other hand, open flowers 

 are formed, because then the competition among the flowers has not become so great, and insects are 

 attracted even by the flowers of the wood-sorrel. The same holds true for species of violet (e. g 

 Viola mirabilis), which * bloom unseen.' General rules regarding the appearance of cleistogamous 

 flowers cannot be deduced from these examples, but each case must be studied separately. 



