422 Mr Gustav Mann's 



summer as compared to cells formed in the winter months. 

 In fig. 1, I'late 1 1., I have represented two cells of *S'. nitida ; 

 the shaded portion shows the increase in length that takes 

 place during winter. The cells are drawn by aid of a 

 Nachet's camera. The smaller cell represents the average 

 length in the month of July, while the larger shows the 

 average length of cells in February ; but I shall refer to this 

 fact again later on in my paper. 



The cells at the very foot of each thread show signs of 

 decay, for the chlorophyll bands tend to be irregularly 

 disposed and to become vacuolated. As these cells decay 

 they will not be able to support the weight of the whole thread, 

 they will collapse therefore and form the already mentioned 

 brownish material ; but this change is so gradual that, under 

 normal conditions, the foot will be strong enough to moor 

 the whole thread to the bottom of the pond. 



To estimate the amount of water, combustible and non- 

 combustible solids, I proceeded thus : Healthy Sjpirogyra 

 material {S. nitida and 8. jugalis in about equal propor- 

 tions) was repeatedly washed to remove all impurities, as 

 bits of decayed matter, water insects, &c. ; then the material 

 was washed in distilled water to remove the salts which are 

 in ordinary water, placed on muslin in a large filter and 

 allowed to drain till it assumed a light green colour, showing 

 that the water between the threads had run off. One 

 kilogramme of this drained material was placed in a large 

 shallow porcelain dish, the weight of which had been 

 previously ascertained, and the dish exposed on a sand-bath 

 to a temperature of 200° F. for 48 hours, then the tempera- 

 ture was raised to 230° degrees F. for 6 hours. The 

 Sjnrogi/ra material had formed a hard crust in the dish, and 

 as soon as the dish had cooled it was weighed ; for if the 

 weighing be delayed for several hours, the dried material will 

 increase considerably in weight from condensation of the 

 atmospheric moisture. The result of the weighing showed 

 that of tlie original 1000 grms., 968 grms. had been eva- 

 porated, giving a percentage of 96*8 of water, and a 

 residue of 32 grms. of solids. The residue was placed in a 

 platinum vessel and burnt, to calculate the amount of pure 

 ash ; after burning, the amount of ash equalled 4"8 grm., the 

 volatile substances therefore equalling 27'2 grm. for 1000 



