210 Dr. F. E. Fritsch. On the Subaerial and [Jan. 2, 



cases this incrustation is due to carbonate of lime, but in other cases it is not 

 affected by acids, and it has as yet been impossible to determine its exact 

 nature. It seems plausible to associate this incrustation with the presence of 

 the air, and possibly the latter may be a result of the formation of the 

 former. The phenomenon is certainly no casual one,* as it is to be met with 

 quite frequently in different parts of the island ; the species exhibiting it are 

 in part common forms throughout the lowlands, and are by no means always 

 found with the associated air. 



The third type of growth is the " tufted " one,f and is characteristic not 

 only of many of the Cyanophycese, but also of the species of Trentepohlia.\ 

 In each tuft the filaments are firmly intertwined with one another to form 

 bundles (fig. 1), which stand out approximately vertically from the sub- 

 stratum. The free ends of the filaments thus project outwards, and are 

 occasionally considerably elongated and practically unbranched (frequently 

 in Scytonema and Tolypothrix) ; sometimes the projecting ends are of a 

 different (generally brighter) hue than the remaining parts of the filaments. 

 The tufts are often (notably in Schizothrix, fig. 1, D) so closely placed that, if 

 they all happen to be of the same length, it is difficult without the aid of a 

 lens to determine the composite character of the growth ; on the other hand, 

 in the genus Symploca the tufts are generally quite isolated from one another. 

 Where a rich tufted growth occurs, the substratum presents a velvety 

 appearance owing to the numerous vertical tufts of intertwined filaments, 

 and this pilose covering is like a wet sponge in the rainy season. The least 

 pressure squeezes water out of it. The tufted type of growth constitutes the 

 prevalent habit in Symploca, Schizothrix (fig. 1, D), etc., and a number of forms 

 which assume the tangled type of growth at first, subsequently raise their 

 filaments into tufts ; such are Tolypothrix, Scytonema (fig. 1, C), Stigonema, etc., 

 especially the species of the first-named genus varying very much in their 

 manner of growth. 



* In a few cases, encrusting unicellular forms were found to exhibit the same charac- 

 teristic association with a film of air, although not growing in protected situations ; the 

 forms involved are species of Glceothece. Here also the thick outer lamellae of the sheaths 

 in most of the colonies were provided with a thin incrustation of a substance whose 

 nature I have been unable to determine. Under the microscope, one or more air-bubbles 

 are seen to be associated with each colony, being frequently drawn out into a kind of beak, 

 the apex of which is in very close connection with the sheath of the Alga. 



t I must point out that the terms " tangled," " tufted," etc., refer to the macroscopic 

 appearance of the Alga. Under the microscope, many Algae of the adhesive type show a 

 tangle of filaments and, in the tufted form of growth, the filaments in each tuft are 

 entangled with one another to a varying extent. 



% Some of the species of Trentepohlia exhibit a slight tangled growth at first, while the 

 basal disc (" sole ") is, of course, always adhesive. 



