REJUVENESCENCE IN NATURE. 223 



The gradual changes which take place in the water- 

 nets preparing for fructification, converting the contents 

 of the vegetative cell into thousands of gonidia, must be 

 observed by night; but it sometimes happens that the 

 particular cells do not advance far enough in the dis- 

 solving preparation, in the course of the night, to enable 

 the influence of the morning's light to complete the 

 formation. Such cells exhibit the earlier preparatory 

 stages, otherwise rapidly passed through in the course of 

 the night, with especial distinctness. Some thus remain 

 all day without any perceptible alteration, completing the 

 preparatory stages in the second night, and maturing the 

 gonidia on the second morning. 



As a second instance, I will bring forward Ulothrix 

 zonata, a Conferva frequent in the mountain brooks of 

 the Schwarzwald, descending also into the plains in the 

 Dreisarn. The mode of growth and the formation of 

 gonidia have already been examined (pp. 148, 1 59). If a 

 little tuft of this Alga is placed under the microscope in 

 the morning, filaments of two characters are seen ; some 

 (still vegetating) with parietal contents of the clearest, 

 brightest green, leaving the ends of the cells almost free, 

 in which green contents exist scarcely perceptible point- 

 like granules, a few small starch-globules, and a large 

 likewise parietal nucleus, which it is not easy to see ; the 

 others (in the condition of fructification) of somewhat 

 torulose aspect, and with more concentrated, darker 

 contents, divided into 4, 8, or 16 masses (gonidia). We 

 rarely find transitions from one structure to the other, 

 namely filaments with darker and more opake green 

 contents not yet divided, in which the fine granules are 

 more strongly developed, but the starch-granules either 

 lost, or only their little nuclei visible, because the 

 transitional stages are usually passed through in the 

 night, and the formation of gonidia completed early in 

 the morning. Birth and swarming of the gonidia, which 

 latter mostly lasts more than an hour, may be observed 

 all the morning. All the mature cells are usually emptied 



