CAUSES OF DEATH 463 



always lie outside the properties of the protoplasm. In 

 annuals and biennials it would seem to be nutritive. It 

 can be readily seen that, as a perennial of the type of a forest 

 tree waxes old, the difficulties of supply to the buds, reaching 

 further and further from the root system, must increase. 

 In its heart-wood, dead and functionless except as a support, 

 the tree bears within it a source of danger ; bacterial and 

 fungal infection through broken branches can spread more 

 readily in dead tissue, destroying the supporting material, 

 and from there may invade the living regions. The tree 

 becomes more susceptible to disease, and less able to 

 withstand the force of the wind. If, after a life of centuries, 

 it must ultimately fall, we may still ascribe its death, not to 

 the ageing of the protoplasm, but to effects, in their essence, 

 secondary. Discussions of this difficult subject will be found 

 in Arber, Molisch (1921), Child (191 5), Doffiein (19 19), 

 and Pearl (1922). 



We may conclude by drawing attention to the beautiful 

 illustration of the roles and relative importance of vegetative 

 multiplication and sexual reproduction offered by the 

 practice of the gardener and breeder. The stock of a plant 

 is rapidly and easily increased by propagating it from 

 cuttings ; and by this means the gardener makes sure of 

 a uniform progeny. He knows that if he raises a fine 

 snapdragon from seed, he may get a bright mixture of 

 offspring, due to crossing, but that if he uses cuttings he 

 will preserve his fine variety true. The breeder looking 

 for new varieties uses seed. " Bud " sports do crop up 

 now and again, e.g. those which originated the weeping 

 varieties of trees, but they are infrequent in comparison 

 with the mutations, which are prepared for in the intimate 

 processes of nuclear division prior to reproduction, and 

 which appear in the offspring from seed. Here, too, is 

 given the further and most valuable power of redistribution 

 and combination in hybridising. Evolution in nature and 

 the art of breeding may be said to depend on sexual repro- 

 duction. Vegetatively the plant multiplies, but remains 

 the same ; through sex it changes. 



