SUPPLEMENT 109 



As in these two cases so also in numberless Algae and Fungi, two methods 

 of reproduction have been discovered, viz. sexual, i. e. by fertilized ova, and 

 asexual, i. e. by swarmspores. Not infrequently it happens that in certain 

 species several types of reproductive organs make their appearance, all of which 

 subserve asexual or ' vegetative ' propagation. 



1. 52 P. 358, 1. 24, for It is owing . . . permit of more, read More especially 

 in Saprolegnia, vegetative growth, formation of swarmspores, and then sexual 

 reproduction follow one after the other, under natural conditions, with such 

 regularity that we might well conclude that the cycle of development was 

 unchangeable. Thanks are due primarily to G. KLEBS (1889; summary in 

 1896 and 1904) for his efforts, carried out over many years, to extend and 

 improve our knowledge of this subject. The principal result of his labours 

 may be briefly expressed by saying that the normal cycle of development of an 

 alga or a fungus represents only one of many possibilities, and that it is often 

 possible by altering the culture conditions to induce a definite change in the 

 cycle, with the certainty of a chemico-physical experiment. External factors 

 determine, in other words, which of the different developmental possibilities 

 which the organism exhibits under natural conditions shall show itself. Even 

 the normal cycle is determined by external conditions, and so long as these 

 remain constant the organism goes on growing in the usual way. 



KLEBS (1903, p. 41) has succeeded in inducing variations in the life cycle 

 of Saprolegnia mixta in the following way : 



1. The mycelium grows for years in a purely vegetative manner if sup- 

 plied continually with fresh and suitable nutrients. 



2. If such a w r ell-nourished mycelium be transferred to pure water it 

 proceeds rapidly to form abundant sporangia. 



3. In solutions of leucin (o-i per cent.) and haemoglobin (o-i per cent.) 

 active growth first of all takes place, followed by the formation of sexual organs. 

 Swarmspores are not formed, but these make their appearance subsequently 

 to the sexual organs if a more dilute solution of haemoglobin (o-oi per cent.) 

 be employed. Similar experiments with Vaucheria repens give the following 

 results : 



1. Continuous vegetative growth took place in fresh inorganic nutritive 

 solutions in bright light, and even in weak light when the nutritive solutions 

 were seldom changed. 



2. Continuous zoospore formation until the thallus was completely 

 exhausted, when placed in darkness after being previously well nourished in 

 water, in light. 



3. Regular alternation of growth and zoospore formation when grown in 

 water, in alternate light and darkness. 



4. Growth and continuous oosperm formation when cultivated on moist 

 soil in light. 



5. Growth, then swarmspore formation and later oosperm formation, 

 if grown first in a nutritive solution and then placed in water in bright light. 



6. Growth, then oospore and, later, swarmspore formation, if grown 

 first on damp soil, then in water or a dilute nutritive solution. 



From these experiments it is evident that in nature also the life-history 

 must vary from the normal in accordance with changes in the environment, 

 and this has actually been observed to take place. 



KLEBS, partly in conjunction with his pupils, has investigated a large 

 number of Algae and Fungi in the same way as in the case of the examples 

 quoted. 



The principal result arrived at was in all cases the same, viz. that the 

 developmental cycle was in no sense determinate or invariable. In every case 



