SECT. 11 PHYSIOLOGY 275 



ever, easily done with tlie Willows, Avhen branches are used as cuttings. 

 What is here artificially induced takes place in other ways in nature. 

 Many rhizomes continue to grow at their ends in the ground, form 

 new roots at some distance from the apex and so persist, while the 

 older portions die away. Strictly speaking we are not dealing here with 

 ordinary growth but with the simplest kind of reproduction involving 

 the production of new living units consisting of root and shoot. When 

 in addition to the straightforward gi-owth of the apex a rhizome 

 also exhibits branching, increase in number of individuals is associated 

 with this reproduction ; from one living unit several are produced. 



Reproduction in lower plants, which exhibit no differentiation, is 

 efiected by each cell division. Since, however, cell division occurs 

 when the amount of protoplasm has doubled, reproduction is in this 

 case identical with the new formation of protoplasm. Only those 

 forms of reproduction require special consideration in which special 

 organs are formed (reproductive, organs, germs) which separate from 

 the parent plant and, at the expense of a supply of reserve material, 

 commence a new life. In this way young organisms originate which 

 then repeat the development of the parent orgarnsm, its gradual 

 increase in strength and its later decay. Often these reproductive 

 organs have the further duty of carrying the organism over a period 

 of cold or drought ; they thus constitute a resting stage. With 

 favourable conditions their growth recommences, they germinate. 



The conditions of the outer world make the still further demand 

 upon reproduction, that from it a multiplication of the species should 

 result. As the germs after separation from the mother plant do not 

 always find the conditions necessary for their development, and so for 

 the most part perish, the extinction of the whole species would soon 

 result if a plant produced but a single germ. In reproduction care is 

 taken for the multiplication of the individual in an almost spendthrift 

 manner. The number of germs produced is a hundred, a thousand, 

 or even millions of times that of the parents. Innumerable spores 

 are produced by a single mushroom, or by a large fern. Bessp]Y has 

 estimated the number of seeds produced annually by a Poplar tree at 

 about 28 millions. 



Rejuvenation, separation, and multiplication of the individual 

 are accordingly the essential requisites of reproduction. It is also 

 important that the germs should be distributed as widely as possible. 



These requirements are fulfilled by plants in the most varied 

 manner. Each great division of the vegetable kingdom has adopted 

 its own special method ; and each family and genus, or even the 

 different species, are characterised by some peculiar feature of their 

 manner of reproduction. Systematic botany is, indeed, essentially 

 based upon diflferences in the development of the reproductive organs 

 and in their functions. 



Numerous and varied as the processes of reproduction in the 



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