176 PART III. PHYSIOLOGY. [ 41 



culacese and Liliales), they attract insects for the purpose of cross- 

 pollination. 



The petals are brightly-coloured in most flowers, and it is their 

 special function to attract insects. Not uncommonly they are 

 specially modified as nectaries (e.g. Helleborus), and thus further 

 contribute to ensure the visits of insects. 



The perianth-leaves (and sometimes also the bracts), are often 

 capable of performing movements leading to the opening and 

 closing of the flower or inflorescence : thus the flowers of the 

 Crocus, Tulip, and Poppy, and the inflorescence of the Daisy, open 

 under the influence of rising temperature and increasing intensity 

 of light, closing under the contrary conditions : the closing is a 

 protection of the essential floral organs against cold and wet ; it 

 is essentially similar to the nyctitropic movements of foliage- 

 leaves. 



b. Sporophyllary Leaves. As already stated (p. 55) the sporo- 

 phylls are the essential organs of the flower, when they are aggre- 

 gated on a special shoot, and have, in any case, the function of 

 asexually producing the spores. They are more or less generally 

 modified in form and structure in connexion with this function ; 

 and in the many different forms of flowers these leaves present 

 remarkable special adaptations which mainly refer to the process 

 of pollination, to the distribution of the seed, etc. It is impossible 

 to enter upon a further consideration of the biology of the flower, 

 but the phenomena of movement presented by the essential floral 

 organs deserve special mention. Thus the two lobes of the stigma 

 (e.g. Mimulus, Bignonia, Martynia), close together on being 

 touched : the movement doubtless ensures the adhesion of the 

 pollen brought by an insect. The stamens are irritable in many 

 plants. For instance, in Berberis, when an insect touches the 

 irritable base of one of the nearly horizontal stamens, the stamen 

 rises up on its point of attachment as on a hinge, and strikes the 

 insect with the anther, thus dusting it with pollen. Again, the 

 syngenesious stamens of Centaurea shorten on stimulation by 

 touch : the flower is protandrous ; consequently, as the filaments 

 contract, the pollen shed by the coherent anthers is pushed out of 

 the open end of the anther-tube by the style within, and is re- 

 moved by the insect. 



