326 BOTANY PART i 



Nicotiana tabacum virginica apetala, which arose in a culture of KLEBS, 

 must be placed here. In many plants reddish - leaved forms have 

 arisen as mutations. All these forms are distinguished from the 

 parent form in a single character. Once they have arisen they have 

 remained constant in all their descendants. 



Nothing certain is known as to the causes of mutations. If they 

 should prove in certain cases to be determined by external factors they 

 would still be sharply distinguished from modifications. In the mutation 

 a change in the determinants has occurred ; either old determinants 

 have been lost or new ones have made their appearance. The latter case 

 must, however, be rare. Mutations do not appear only in relation to 

 sexual reproduction. Thus in some Bacteria which increase in number 

 by repeated division mutations have been found. In higher plants also 

 single buds are known to have become changed and their new characters 

 have persisted. These cases are spoken of as bud mutations. Doubt is 

 often expressed as to whether in the Bacteria and in the moulds there 

 is any sharp distinction between mutations and modifications. 



Origin of Species. Various lines of evidence, dealt with on 

 p. 206 ff., have led to the view that the organisms which inhabit the 

 earth at the present time have developed from others that existed 

 in previous ages. This hypothesis, which is known as the THEORY^OF 

 DESCENT ( 87 ) and is of great importance, assumes that the " species " r is 

 not constant but liable to change. In addition to what has been said 

 earlier (p. 206 ff.) it is only necessary to state here that only mutations 

 and combinations among the variations yet observed could play a 

 part in the origin of a new species. Latterly the indications that 

 hybridisation has been of importance in the production of species 

 have multiplied. Certain species of Oenothera behave like hybrids 

 the parents of which are no longer in existence. 



SECTION III 

 MOVEMENT 



Phenomena of movement are met with in the living plant not less 

 generally than those of metabolism and development. Metabolism is 

 associated with a continual movement of the raw food-materials, which 

 are absorbed, and of the elaborated assimilates and excreted sub- 

 stances. These movements cannot be directly observed, but are not 

 less certainly established ; they have already been dealt with. In 

 addition there exist a number of visible alterations of position exhibited 

 either by the whole plant or by its several organs ; these movements 

 are, it is true, often very slow but sometimes are quite sudden. 



PROTOPLASM itself is capable of different movements. Naked 

 protoplasmic bodies almost always show slow movements resulting in 



