356 METAMORPHOSIS 



place, but also that reproductive organs are formed only under definite con- 

 ditions, and that the nature of these organs is not determined by internal 

 causes alone. Internal factors, here as everywhere, play, doubtless, an important 

 part, inasmuch as they bring about the state of activity of the plant which 

 enables it to react to the outer world in definite ways ; the environmental factor 

 acts merely as a releasing stimulus. 



Let us now examine more in detail the phenomena of reproduction as 

 seen in Ulothrix. This alga grows in flowing water, in good light and at low 

 temperatures. For these reasons it is not well adapted for laboratory research, 

 and hence Klebs's attempts to elucidate its reproductive physiology were not 

 altogether successful. He was unable to induce the alga to form swarmspores or 

 gametes at will, but he managed to determine that external factors did play a part 

 in the question. Under favourable natural conditions the alga produces no re- 

 productive organs, it simply grows vegetatively, and its filaments become 20 to 

 30 cm. in length. The principal inducement to the formation of zoospores 

 appears to be the cessation of the flow of water and a diminution of oxygen 

 in it. Such a condition of things takes place periodically in regions inhabited 

 by Ulothrix, and hence for the most part growth and swarmspore formation 

 go on at the same time. The alga may exist in this condition for years ; 

 gametes are formed only under special circumstances, apparently when the 

 water level falls and when the cells come directly in contact with air, or are 

 only occasionally sprinkled with water. The zygote can withstand drought, 

 and hence it is intelligible that they should be formed under dry conditions. 



Klebs {1896) obtained more satisfactory results with species of Oedo- 

 gonium, which are more easily cultivated than Ulothrix. The two species 

 investigated behaved differently, but one can only say that both demanded 

 a certain alteration in the surroundings before swarmspore formation could be 

 guaranteed. In 'the case of Oedogonium capillare darkening had a quite marked 

 effect, which was increased by cultivation in a 4 to 10 per cent, solution of cane 

 sugar. After being cultivated in ordinary water, in which it assimilates 

 vigorously, it produced swarmspores on being transferred to a dilute nutrient 

 salt solution. In Oe. diplandrum, on the other hand, swarmspores were formed 

 when it was transferred from flowing to still water, from a lower to a higher 

 temperature, from an inorganic nutrient solution to pure water. This is all 

 the more remarkable seeing that both forms grow in similar localities. It is 

 very desirable that these two plants should be more closely studied under natural 

 conditions, so that we may get to know under what circumstances the forma- 

 tion of swarmspores takes place in nature ; at present we are acquainted with 

 the artificial conditions only. Sexual organs may be produced in both species 

 if the water be limited in amount, and if it contains little in the way of nutrient 

 salts, and if the plants be strongly illuminated. Oedogonium diplandrum re- 

 quires a much greater intensity of light than Oe. capillare. 



Sexual reproduction is by no means essential tothepersistenceof Oedogonium, 

 for Klebs has shown that Oe. diplandrum grows luxuriantly in many situations 

 without ever forming sexual organs, and that only the male form of Oe. capillare 

 occurs near Basel, thus entirely precluding the formation of zygotes. The 

 biological distinction between zygotes and swarmspores is perfectly obvious. 

 By means of the latter the plant is able to reach new locations and by the former 

 to tide over periods when external conditions are unfavourable to vegetative 

 growth. Put shortly, swarmspores subserve distribution, zygotes, continuity 

 of the species. Why a fusion of two cells is necessary for the formation of a 

 resting spore and why these two cells should exhibit sexual differences is not so 

 apparent. On reviewing the reproductive processes in Algae and Fungi we 

 find that although very often sexual fusion is followed by the formation 

 of resting spores, still there are cases of resting spores produced asexually 

 (e. g. Bacteria), and, on the other hand, products of sexual union which develop 



