316 BOTANY PART i 



remo.val of an arrest of growth, though it cannot be said that this was 

 its original and essential significance. It is much more probable that the 

 sexual cells have gradually lost the capacity of independent development 

 since in this way the possibility of fusion was increased. If every 

 sexual cell commenced to grow at once, this would in most cases take 

 place before fusion with another sexual cell could be effected 



This assumption is supported by the behaviour of some Algae, in which 

 the sexual cells can often germinate independently ; the egg-cells especially may 

 develop without fertilisation. From the analogy with similar cases in the animal 

 kingdom this phenomenon has been termed PARTHENOGENESIS. In the primitive 

 Algae parthenogenesis is possible, because in them the incapacity of development 

 of the egg-cell has either not been acquired or is easily removed under special con- 

 ditions. Thus for example in the Alga Protosiphon parthenogenetic development 

 is induced by a high temperature, and the same happens in the case of the ova 

 of some lower animals (Echinoderms) on treatment with solutions of a certain 

 concentration. . It may perhaps be assumed that in the cases in which development 

 only takes place after fertilisation the stimulus to development is given by some 

 substance contained in the sperm-cell. 



Among the higher plants also phenomena to which the name parthenogenesis ( 79 ) 

 has been applied occur ; they are better termed apogamy. Thus the egg-cells of 

 some Compositae, and also of Alchemilla, Thalictrum purpurascens, Wickstroemia 

 indica, Ficus hirta, Marsilia Drummondii, and Chara crinita develop without 

 previous fertilisation. These cases are distinguished from those just described by 

 the egg-cells in question having retained the number of chromosomes characteristic 

 of vegetative cells. They are diploid cells (p. 203) and not fitted for fertilisation. 



We thus arrive at the conclusion that the essential of sexual repro- 

 duction cannot consist in the removal of the arrest to development 

 of the sexual cells. This leads us to consider THE FUSION OF THE 



SUBSTANCE OF THE TWO CELLS AND THE MINGLING OF PATERNAL AND 

 MATERNAL CHARACTERS WHICH FOLLOWS FROM THIS. This brings 



out the chief distinction between the two modes of reproduction ; the 

 vegetatively produced progeny are due to no such mingling of 

 characters. The complex of characters in vegetative multiplication 

 does not differ as a rule from that in the parent form. As a matter 

 of fact, we preserve by vegetative multiplication all the varieties and 

 races of our cultivated plants, even when these do not come true from 

 sexually produced seed. In contrast to the vegetative progeny the 

 sexually produced descendants, as a rule, cannot completely resemble 

 the mother plant, but must combine the characters of both parents. 

 The more these differ from each other, the more striking will be the 

 visible effect of fertilisation. 



F. Heredity, Variability, Origin of Species 



Heredity ( 80 ). By inheritance is understood the familiar pheno- 

 menon that the properties of the parents are repeated in their 

 progeny. This phenomenon is presented to us in the division of a 



