66 SOUTH AFRICAN FLOWERING PLANTS. 



How readily a species will change is often well 

 seen under cultivation. Thus there are two species of 

 Ar'abis {Crucifcrce) in Switzerland, one of which lives 

 in crannies of rocks, and lias thin "papery" leaves, and 

 is called ^. anchoret' ica ; the other has thicker leaves, and 

 is known as A. alpi'na. When, however, the former 

 was grown in the public gardens at Kew, it assumed 

 all the features of the latter. As another example, 

 there is a spiny plant known as " Eest-harrow " in 

 England, common in dry, waste places, called Oiio'nis 

 qnnu'so, ; but plants raised from seed in a border kept 

 constantly moist ceased to produce spines, and assumed 

 the characters of the spineless species, 0. rc'i)cns. 



Plants by the seaside often have somewhat fleshy 

 leaves ; but if they be grown far away inland, they will 

 sometimes produce thin leaves like other plants of the 

 same neighbourhood; and if garden- cress, etc., be watered 

 with salt and water, their leaves then become Heshy, 

 resembling maritime plants. 



Many other such illustrations could be given ; but 

 these will be sufficient for the reader to understand 

 that new varieties and species come into existence by 

 the xjowcv of ada])tation which the living i^rotoplasin ^ 

 of plants possess, in response to the direct action of the 

 external conditions of life. 



It is in consequence of this that all water plants of 



^ This is the name given to the living substance of animals and 

 plants. 



