118 Chlorophycece 



In diffused daylight they place themselves at right angles to the 

 direction of the incident rays, but in strong sunlight the edge of 

 the plate is directed towards the light. This has long been known 

 to students of Alga3 and special mention of it was made by Bennett. 

 Quite recently this phenomenon, which is well exhibited by the 

 chloroplasts of many green plants, has been further investigated 

 and it has been shown that the chloroplast occupies on an average 

 about 30 minutes in rotating through 90 . 1 Apart from the lining 

 layer of protoplasm and the chloroplast, a considerable proportion 

 of the cell-cavity is usually occupied by large fluid vacuoles. 



Vegetative multiplication frequently occurs by the dissociation 

 of a filament into its constituent cells, each of which forms a new 

 plant by rapid cell-division. 



Asexual reproduction takes place occasionally in Mougeotia by 

 the formation of spores resembling aplanospores 2 . These spores 

 are produced by the division of a vegetative cell and they may be 

 regarded as carpospores formed from sporocarps (consisting of three 

 cells) produced without conjugation, but possibly in consequence 

 of the stimulus which has already caused conjugation to take place 

 in a distant part of the filament. In the genus Gonatonema repro- 

 duction is solely by the formation of aplanospores, the whole of the 

 contents of a single cell being generally, but not always, utilized 

 in the formation of a spore. It has been noticed in the three best 

 known species of Gonatonema (viz. G. ventricosum Wittr., G. Bood- 

 lei W. & G. S. West, and G. tropicum W. & G. S. West) that 

 during the formation of the spore, and just before the rounding off 

 of the protoplasmic mass, there is sometimes a more or less 

 complete division of the cell-contents into two parts 3 . I have care- 

 fully studied the formation of aplanospores in this genus and this 

 curious separation of the cell-contents into rudimentary gametes 

 is by no means of frequent occurrence. It is most likely a slight 

 retention of the last traces of ancestral sexual characters, the 

 spores having arisen at one time by a process of conjugation. 



Sexual reproduction of a low type takes place in Mougeotia by 

 conjugation. This almost always occurs between the cells of 

 different filaments which are lying side by side. Each cell puts 



1 F. J. Lewis in Ann. Bot. xii, 1898. 



2 Wittrock, 'Om Gotl. och 01. Sotv. Alg.,' Bihang till K. Sv. Vet.-Akad. Handl. 

 Bd 1, no. 1, 1872, t. ii, f. 7 and 8. 



3 W. & G. S. West in Ann. Bot. 1898, xlv, p. 39, t. iv, f. 3; Trans. Boy. Irish 

 Acad. xxxii, sect. B, 1902, p. 17, t. i, f. 5. 



