322 
DARWINISM 
CHAP. 
sandwort (Spergula), and some willow-herbs (Epilobium); or 
they arch over the pistil, as in Galium aparine and Alisma 
Plantago. The style is also modifiecl.to bring it into contact 
with the anthers, as in the dandelion, groundsel, and many 
other plants. 1 All these, however, may be occasionally cross- 
fertilised. 
3. In other cases precautions are taken to prevent cross¬ 
fertilisation, as in the numerous cleistogamous or closed tiowers. 
These occur in no less than fifty-five different genera, belonging 
to twenty-four natural orders, and in thirty-two of these genera 
the normal flowers are irregular, and have therefore been 
specially modified for insect fertilisation. 2 These tiowers appear 
to be degradations of the normal flowers, and are closed up by 
various modifications of the petals or other parts, so that it is 
impossible for insects to reach the interior, yet they produce 
seed in abundance, and are often the chief means by which 
the species is continued. Thus, in our common dog-violet the 
perfect flowers rarely produce seed, while the rudimentary 
cleistogamic flowers do so in abundance. The sweet violet also 
produces abundance of seed from its cleistogamic flowers, and 
few from its perfect flowers ; but in Liguria it produces only 
perfect flowers which seed abundantly. No case appears to 
be known of a plant which has cleistogamic flowers only, but 
a small rush (Juncus bufonius) is in this condition in some 
parts of Russia, while in other parts perfect flowers are also 
produced. 3 Our common henbit dead-nettle (Lamium amplex- 
icaule) produces cleistogamic flowers, as do also some orchids. 
The advantage gained by the plant is great economy of 
specialised material, since with very small flowers and very 
little expenditure of pollen an abundance of seed is produced. 
4. A considerable number of plants which have evidently 
been specially modified for insect fertilisation have, by further 
1 The above examples are taken from Rev. G. Henslow’s paper on “ Self- 
Fertilisation of Plants,” in Trans. Linn. Soc. Second series, Botany , vol. i. 
pp. 317-398, with plate. Mr. H. 0. Forbes has shown that the same thing 
occurs among tropical orchids, in his paper “ On the Contrivances for insuring 
Self-Fertilisation in some Tropical Orchids,” Journ. Linn. Soc., xxi. p. 538. 
2 These are the numbers given by Darwin, but I am informed by Mr. 
Hemsley that many additions have been since made to the list, and that 
cleistogamic flowers probably occur in nearly all the natural orders. 
3 For a full account of cleistogamic flowers, see Darwin’s Forms of Flowers, 
chap. viii. 
