Form of H or midium flaccidum, A . Braun . 519 
Hormidium flaccidum, the parts affected, namely the longitudinal walls and 
the septa, were carefully examined. These investigations were made in the 
first place on material in which splitting had been abundantly produced by 
supplies of water alternating with periods of desiccation (p. 516), and which 
had been subsequently kept without water for six weeks. 
The longitudinal walls were relatively thin, but one could usually dis- 
tinguish a ‘ cuticle * of variable thickness, though undergoing disintegration 
here and there, especially in the neighbourhood of the septa. In curved 
filaments the cuticle appeared folded or corrugated along the concave surface 
of the bend (Fig. 3, A). Occasionally isolated bulbous thickenings were 
deposited on the inner surface of the longitudinal wall, while in dead cells 
a definite, though irregular, inner layer was frequent. The septa were 
commonly biconcave owing to the two component lamellae diverging before 
merging into the longitudinal wall, the region between them being occupied 
by a substance of different refractive index (Fig. 3, b). The latter often 
extended right across the septum so as to separate completely the two 
lamellae, and had occasionally become drawn out in the longitudinal 
direction so as to form a short cord connecting the two adjacent cells 
(Fig. 3, c, m.). Even when the thickening was much less pronounced the 
filaments were often slightly constricted in the regions of the thickened 
septa. 
Thickened biconcave septa have been observed in desiccated material 
growing naturally out of doors in cold and warm weather. In such cases, 
however, the amount of damping produced by dew during the time of 
drought, as well as the exact external conditions preceding it, have not been 
properly known. The fact that thickenings did not appear in material 
subjected to continuous desiccation during many weeks when the dew was 
inconsiderable (cf. Desiccation Experiment I, p. 525) seems to indicate that 
drought is not the entire cause of this particular thickening of the septa. 
There may be a certain amount of intermittent moisture required, and 
possibly individual peculiarities of adjacent cells have some additional 
influence, as is suggested by the fact that septa in the same filament exhibit 
a great deal of variation in the degree of thickening, many remaining 
unthickened while others are considerably thickened. 
Some indication of the chemical nature of the longitudinal walls and 
septa was sought by treatment of the above-mentioned material with various 
reagents and stains. Walls and septa were completely dissolved by con- 
centrated sulphuric acid. Ordinary iodine solution, added to fresh filaments, 
caused the ‘ cuticle * to become very distinct as a dark line, which was 
thinner opposite the septa, the disintegrated portions staining blue. The 
remainder of the longitudinal wall, as well as the septa, assumed a pale blue 
colour. Methylene blue stained the disintegrating parts of the ‘cuticle* 
(Fig. 3, d) and the debris of thickening substance often associated with 
