374 EFFECTS OF CULTIVATION ON OEGANS. 



Cultivation has a great effect in causing changes in the various 

 parts of plants. Many alterations in form, size, number, and adhesion 

 of parts, are due to the art of the horticulturist. The development 

 of cellular tissue and of starchy matter is often thus much increased, 

 as may be seen in the case of Turnips, Carrots, and Potato. The 

 succulence of the leaves of the Cabbage and Lettuce, and the forma- 

 tion of a heart, as it is called, is due to cultivation ; so also the curled 

 leaves of Savoys, Cress, Endive, etc. The changes in the coloilr and 

 forms of flowers thus produced are endless. In the Dahlia, the 

 florets are rendered quilled, and are made to assume many glowing 

 colours. In Pelargonium the flowers have been rendered larger and 

 more showy ; and such is also the case with the Eanunculus, the Au- 

 ricula, and the Carnation. Some flowers, with spurred petals in their 

 usual state, as Columbine, are changed so that the spurs disappear ; 

 and others, as Linaria, in which one petal only is usually spurred, are 

 altered so as to have all the petals spurred, and to present what are- 

 caUed pelorian varieties. 



Section IV. — Some General Phenomena connected with 

 Vegetation. 



1. — Vegetable Irritability. 



Under this head are included certain sensible movements of living 

 plants not referable to mere elasticity, or to the hygroscopic nature of 

 the tissues. These motions are influenced chiefly by light and heat, 

 and, like many phenomena occurring in organised beings, they cannot 

 at present be fully explained by chemical or mere mechanical laws. 

 They may, however, be excited by stimuli of a chemical or mechanical 

 nature. Although the cause of them is obscure, stUl, in some in- 

 stances, their use is obvious. 



Among the lowest classes of plants there are some peculiar move- 

 ments of this kind. The simplest members of the searweed tribe- 

 occasionally move throughout their whole substance. OscUlatorias, 

 which are filaments composed of cells placed end to end, containing 

 fluid and granular matter, have an undulating movement, by means 

 of which they advance. When placed in fluids under the field of the 

 microscope, some of them may thus be seen to pass from one side to 

 the other. The filaments sometimes twist up in a spiral manner, and 

 then project themselves forward by straightening again. The motions 

 are influenced by temperature and light. The spores of many Crypto- 

 gamic plants, especially species of Vaucheria, and Conferva, and 



