i893. HYBRIDITY IN PLANTS. 209 



wheat and rye belonging to different genera. Between different kinds 

 of barley he obtained two artificial and six natural hybrids ; in no 

 case did he succeed in fertilising a two-rowed barley by pollen of a 

 variety with a larger number of rows. Among oats, five natural but 

 no artificial hybrids were obtained. Very few natural hybrids occur 

 among peas ; with different varieties of beet crossing is much more 

 common. Rimpau states that if a new form exhibits great variability 

 among its descendants, it is probably hybrid ; while, on the other hand, 

 it is most likely a spontaneous variety if the descendants maintain 

 great constancy. 



The Darwinian hypothesis that a sexual mode of reproduction is 

 absolutely necessary to the maintenance of the higher forms of life, 

 and that continual propagation by non-sexual methods must result in 

 deterioration and ultimate extinction, has not been allowed to pass 

 unchallenged during recent years. Mobius in Germany and Meehan 

 in America have pointed out the great length of time, extending in 

 some cases to thousands of years, during which some plants have 

 been continuously propagated by non-sexual methods without apparent 

 deterioration or increased liability to disease. This is the case 

 with many fruit-trees, such as the fig, date-palm, banana, yam, 

 batatas, and olive, and with the Canadian water- weed, Elodea 

 canadensis, imported into this country from America, the male plant of 

 which is still unknown here ; though it is asserted by others that this 

 pest is now gradually dying out from our rivers and canals. On the 

 other hand, the weeping-willow and the Lombardy poplar — varieties 

 which never produce seed, and which are propagated solely by 

 cuttings — do appear to be threatened with extinction, owing to their 

 abnormal liability to disease. 



We now come to the second part of our subject, and, as we have 

 already said, the more difficult one — the nature of the process of 

 fertilisation itself. In general terms, fertilisation, or fecundation, may 

 be defined as the union of the elements of an active male with those of 

 a passive female cell, the result being the production of an embryo 

 which develops into a new individual. In the animal kingdom, the 

 only mode of reproduction in all the higher forms is a sexual one, 

 non-sexual propagation having survived only in the lower types. In 

 the vegetable kingdom, the two modes work side by side in the 

 higher forms, while in the lowest forms sexuality is unknown. It 

 becomes, therefore, a problem of great interest to determine in what 

 way the sexual mode of reproduction arose among plants. Much 

 light is thrown on this question by the phenomena which have been 

 observed among Algae. A very common mode of propagation among 

 our ordinary fresh-water Algae is by means of non-sexual zoospores or 

 swarmspores, minute flagellate bodies which escape from the cells of 

 the parent plant, move about in the water with great rapidity, and 

 then finally come to rest, withdraw their cilia, and develop into new 

 plants. Among the brown sea-weeds, there is a great variety in the 



