ABSTRACTS OF FOUR LECTURES TO STUDENTS 



523 



equally applies to the pendulous flowers of the Currant and Lily of the 

 Valley ; hence such flowers are symmetrical around a central axis ; but 

 irregular flowers are, on the contrary, almost invariably close to the stem, 

 and are always visited on one side, the front only. Hence they are said 

 to be bilaterally symmetrical. 



Various suppressions of some parts, with alteration of forms of others s 

 have taken place in converting a regular flower into an irregular one ; but 

 whatever they are, the result is in perfect adaptation to the habitual 

 visitor. 



Hence it is that, while an open regular flower, as that of a Buttercup, 

 may be visited by scores of insects of different species, an irregular flower, 

 as that of the Aconite, may be visited by only one kind. 



As another feature : In many flowers the anthers mature simultane- 

 ously with the stigmas, so that the pollen, falling on them, pollinates 

 them. This is called self-pollination, and is characteristic of many incon- 

 spicuous flowers ; but the rule for conspicuous flowers is that the pollen is 

 shed before the stigmas of the same flower are ready to receive it. 



The interval varies greatly in different flowers and in different seasons. 

 In many it is so great that every grain of pollen is shed, so that unless 

 the flower be crossed it cannot set seed, but more generally the interval is 

 not so great but that a chance is left for the stigmas to get pollinated and 

 self-fertilisation is the result. 



Even with highly irregular flowers and strongly " protandrous " {i.e. 

 stamens first), as those of the Aconite, Larkspur, Lupin, Foxglove, &c, 

 though specialised for insects, yet in their absence (as in a garden in 

 London) appear to be self-fertile, by reducing the interval between the 

 maturation of the anthers and that of the stigmas. There is another 

 hindrance to self-pollination, with the danger of no seed being set, and 

 that is the formation of an obstruction between the anther and the stigma. 

 This -occurs in most Orchids and is called the rostellum ; so that, unless the 

 pollen be removed by an insect and applied to the stigma, no seed can 

 possibly be set. 



Some few Orchids have acquired the power of self-fertilisation by the 

 non-development of the rostellum ; the pollen then slips down into the 

 " stigmatic chamber " just below it.* 



These facts prove that to be highly specialised is far from being of any 

 advantage to a plant when we remember the only " ends " of plant life 

 are to live and set seed. 



On the other hand, it is the rule that flowers which can be self -pollinated 

 and so self-fertilised are always most abundant, because they set an extra- 

 ordinary amount of seed. 



Hence the above two "ends " are best and infallibly secured by self- 

 pollination. 



An easy experiment will prove this. Let Groundsel, Chickweed, 

 Shepherd's-purse, Black Solanum, Urtica urens, &c. have a free run of a 

 garden. They will soon crowd out and destroy, in the struggle for ex- 

 istence, all the garden flowers or vegetables. It will be soon seen that 

 there is no want of health or vigour about them. 



* " On the Contrivances for insuring Self-fertilisation in some Tropical Orchids," 

 Journ. Linn. Soc. xxi., p. 538. See Origin of Floral Structures, pp. 319, 253 &c. 



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