64 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 Mydrodictyon: subsequently his 
observations were extended to Vaucheria, and later to many other Algae 
and Fungi! Up to 1890, when Klebs’ first paper on Aydrodictyon 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 
‘Klebs, Bedingungen der Fortpflanzung bei etnigen Algen und Pilzen, Jena, 1896, 
