PERIODICITY IN DEVELOPMENT. II 



355 



is only after their fusion that further development can take place ; at the same 

 time, however, there is one point of difference between the two cases, viz. fusion 

 in the plant is followed by a resting period, entirely absent from the animal. 

 It is impossible to doubt, however, that the reproductive cells in Oedogonium 

 have arisen from ordinary swarmspores, and that the plant itself has been derived 

 from forms characterized by the possession of motile gametes which fuse in pairs 

 as is still the case in Ulothrix. These cells were originally alike, and a differen- 

 tiation has gradually appeared in them, to which we apply the term ' sexual'. 



It was only natural that the discovery of sexuality in the lower plants 

 (PRINGSHEIM, 1855), comparable to the only form of reproduction in the higher 

 animals, should have created an immense sensation, and that from that time 

 onward an attempt was always made to determine, in the first instance, in every 

 account of reproduction whether the process was sexual or asexual (vegetative), 

 for it was the custom to refer the extremely varied forms of organisms to one or 

 other of these two categories. This view was, however, incorrect for two reasons. 

 In the first place, it was shown that sexual reproduction had arisen at several 

 points in the plant world and that possibly 

 it had not the same physiological sig- 

 nificance in all groups. Under vegetative 

 reproduction, also, were included a variety 

 of processes which could not well be 

 homologized. It is perfectly obvious that 

 the ' vegetative ' swarmspores of Oedo- 

 gonium have far more points in common 

 with the ovum and spermatozoid than 

 with a slip of a willow, and yet it is still the 

 custom to treat of such swarmspores side 

 by side with ' cuttings ' in a chapter on 

 ' vegetative propagation ' . Unfortunately, 

 it is not possible at present to replace this 

 old-fashioned classification with one based 

 on the knowledge now in our possession, 

 although attempts have already been made 

 in this direction. Thus HANSTEIN (1877) 

 distinguished between reproduction by 

 'embryos' and by 'buds', and MOBIUS 

 (1897) has attempted to carry out this 

 subdivision more fully. Without giving 

 the reasons for the view we hold, it appears to us that this classification does 

 not meet the case, so that we must continue to use the old nomenclature in spite 

 of its defects. We are entitled to look forward to rapid progress in this 

 subject as a result of the renewed interest taken lately in problems connected 

 with the physiology of reproduction (compare KLEBS, 1900 a), although at the 

 same time we must not expect that the infinite variety of reproductive pro- 

 cesses in nature may ever be brought under one uniform system. 



Let us inquire now as to the factors which render reproduction in general 

 possible, and those which are essential to the appearance of the different types 

 of reproduction more especially in these Algae. For a long time it was imagined 

 that these factors were essentially internal, and that in the lower plants, more 

 especially Algae, just as in the higher plants, asexual and sexual reproduction 

 alternated regularly. It was assumed that reproduction was a necessary result 

 of internal development when the plant had reached a certain size or age. It 

 is owing to KLEBS'S (1896 onwards) researches that our insight into this question 

 is now deeper and more exact. The chief result of KLEBS'S investigations is 

 briefly that under appropriate external conditions growth and division only take 



A a 2 



/, Oedogonium boscii. Fertilized 

 oogonium. The ovum is already surrounded 

 by membrane ; above, a spermatozoid. After 

 KLEBAHN (Jahrb. f- wiss. Bot. 24, PL 31. x 

 175. //, Oedogoniunt boscii. Young, just opened 

 oogonium, in and in front of the opening, mucilage. 

 After KLEBAHN (ibid.). X 400. ///, Oedo- 

 goniunt 'landsboroughi. Spermatozoid forma- 

 tion. In the lowermost cell the spermatozoids 

 are forming by bipartition; in the cell above it 

 they are escaping by a fissure in the wall. After 

 HIRN (1900). 



