164 BOTANY PART I 



or biont is found, and the bionts are characterised by differences in 

 form. Thus in the Rust of "\\ r heat (Pucdnia graminis) the individuals 

 which live on the Barberry produce as reproductive organs aecidia 

 and spermogonia. From them arise the quite distinctly organised 

 biont living on the leaves and stems of grasses which is reproduced 

 by means of uredospores and teleutospores. From the latter, on damp 

 soil in the spring, tubular germ-plants arise which in their turn bear 

 a speial kind of reproductive cell, the basidiospores. With this type 

 of spore the developmental cycle of the species is completed, since 

 from the basidiospores are again formed the individuals which live 

 on the Barberry. 



In treating of the cell processes which constitute fertilisation (p. 92), it has 

 already been seen that they result in a doubling of the number of chromosomes 

 present in the nucleus. Doubtless, the process of fertilisation has repeatedly 

 originated in both animal and vegetable kingdoms as soon as a certain grade of 

 organisation had been attained in the course of phylogenetic development. In 

 every case the process of fertilisation must have resulted in a doubling of the 

 number of chromosomes. The single chromosome number must be assumed to be 

 the number originally occurring in the organism. According to the view of most 

 investigators the chromosomes are persisting entities in the nucleus. Further, 

 there is nmch in favour of the view that they are different from one another. 

 Under these conditions the doubling of the number of chromosomes in the act of 

 fertilisation would lead to a progressive increase in their number if the reduction 

 division (p. 84) had not been introduced into the course of development of the 

 organism ; in this division the number is reduced to one half, that is to say, a return 

 is made to the original condition. If in the ontogeny of an organism, the reduction 

 division follows immediately on fertilisation all the bionts of the species will show 

 the single number of chromosomes. This was, in any case at first after the distinc- 

 tion of sex, always the condition of things, and it is found in many existing 

 Thallophytes. In the further course of phylogenetic development the sexual 

 product developed into a special biont, which accordingly had the double number 

 of chromosomes in its tissues. Not until the formation of its reproductive cells did 

 this biont proceed to the reduction division by which the number of chromosomes 

 returned to the original single number. When in the developmental cycle of a 

 species bionts with the single chromosome number and others with the double 

 number are represented, we have to do with an alternation of generations. Such 

 an alternation of generations is found in all Bryophyta, Pteridophyta and Phanero- 

 gams, i.e. in plants from the Bryophytes onwards. The generation with the 

 double number of chromosomes became in these plants the dominant one, while 

 that with the single number underwent a progressive diminution in size. Thus 

 the plant body which is most prominent in Pteridophytes and Phanerogams is the 

 generation with the double number of chromosomes, while the corresponding 

 generation with the single number becomes so inconspicuous that it ultimately 

 requires to be carefully looked for. This is especially the case for the Phanero- 

 gams, in which this generation no longer has an independent existence, but 

 completes its development within the generation with the double number of 

 chromosomes. The generation with the single number of chromosomes is in 

 plants termed the gametophyte ; that with the double number the sporophyte. 

 In order to extend the distinction to the animal kingdom the terminology 



