6 4 ALTERNATION IN THE THALLOPHYTES 



lower in the scale, the case is found to be often different : the first clear 

 demonstration for a Thallophyte that the phases do not follow an obligatory 

 succession was given by Klebs for Hydrodictyon : subsequently his 

 observations were extended to Vaucheria, and later to many other Algae 

 and Fungi. 1 Up to 1890, when Klebs' first paper on Hydrodictyon was 

 published, the comparison of the various stages of life in the Thallophytes 

 and indeed in plants at large had rested on form, and very little was 

 known of their behaviour under varying conditions. But Klebs applied 

 to them the experimental method, and found in many cases that where 

 the organism possesses two or more kinds of propagation, each is directly 

 dependent upon quite definite external conditions. There appears in many 

 cases to be no cause in the inner nature of the organism for one of these 

 to be developed earlier or later : it lies in the hand of the experimenter to 

 determine their succession. An exact knowledge of the conditions gives the 

 experimenter the secure control over the organism, which can at will be 

 forced into any desired mode of propagation within the limits of its species. 

 This may be well illustrated by the case of Vaucheria, which happens 

 to have had its simple life-history adequately investigated through the 

 experiments of Klebs. He found that the formation of the vegetative zoo- 

 spores is most active if well-nourished plants are transferred to fresh 

 external conditions, and especially to diminished intensity of light; while 

 the formation of sexual organs can always be counted on when nutrition 

 proceeds slowly, under relatively equable conditions, and under good 

 illumination. Similar observations, correlating the phases of life of various 

 Algae and Fungi with external conditions, have also been carried out, 

 and though the determining circumstances may differ in different plants, 

 the fact is now demonstrated for a number of Thallophytes, that there is 

 in them no obligatory succession of phases : sexual reproduction or vege- 

 tative propagation may be repeated indefinitely, according to the conditions 

 of life : neither of these leads of inner necessity to the appearance of the 

 other. In fact the relation of the vegetative mode of propagation by 

 isolated germs to the life-cycle of such Thallophytes is somewhat similar 

 to the production of gemmae on the gametophyte of Archegoniate Plants, 

 or to the sporophytic budding of Ferns or Lycopods. In all such cases, 

 which may collectively be styled under the general term of somatic 

 budding, the increase in number of individuals is secured, but only by an 

 incident which takes no part in any rhythmic succession of obligatory 

 phases, and involves no cytological change. The result is simply a repetition 

 of the same phase from which the bud, gemma, or zoospore was itself 

 derived. The experimental method of Klebs, by showing that the order 

 of events in such cases is not obligatory, has laid the foundation for a 

 more rational comparative study of the life-story in the Thallophytes. It 

 may be considered probable that many more of the Fungi and Algae will 

 be found to behave like the species which have been tested. As operative 

 1 Klebs, Bedingungen der Fortpflanzung bei einigen Algen uud Pilsen, Jena, 1896. 



