208 THALLOPHYTES. 



prominent property of the external conformation of most Algae and of all Fungi ; 

 but a sharp boundary line cannot be drawn in this respect between the Algce and 

 the group of Muscinese. On the one side some Algse show a clear differentiation 

 of stem and leaf, while on the other side the vegetative part of many Hepaticae is 

 a very simple thallus. As little can a boundary Hne based on the structure of their 

 tissues be drawn between Algas on the one hand and Characea3 and Muscineae on 

 the other hand. It would be very easy to adduce a series of negative characteristics 

 of Thallophytes ; i. e. to show^ that certain properties are wanting in them which 

 are found in all other plants or single groups ; but this would only prove that 

 Algae and Fungi are neither Characeae, Muscineae, nor Vascular plants, but would 

 tell us nothing respecting their affinity. 



CLASS I. 



A L G ^ \ 



The Algse begin with the smallest and simplest forms of the Vegetable 

 Kingdom ; but they approach in different ways a more perfect development, and 

 attain higher degrees of organisation, with frequently dimensions as considerable as 

 are again found only in much more highly organised classes. This gradual approach 

 towards a more perfect development takes very different courses ; in some cases 

 it is the differentiation of the individual cell, m others the mode of combination of 

 the cells, in others again the external conformation, the variation of growth in different 

 directions, but usually it is a combination of these conditions, by which more perfect 

 structures proceed from simpler ones. The development of the mass of Algae 

 is also attained in different ways ; sometimes the single individuals increase, at 

 other times- numerous individuals are united into a genetic and organic whole, 

 which behaves as an individual. The former occurs especially in the forms which 

 are morphologically and histologically more perfect (Fucaceae), the latter in the 

 simpler structures (Gelatinous Algae, Nostocaceae). 



If we consider, first of all, the Differentiation of the Individual Cell, we find in 

 the lowest grades of this class forms in which scarcely more can be recognised 

 than a cell-wall containing a coloured protoplasmic substance, the latter always 

 possessing a vacuole. Higher forms show the protoplasmic substance differentiated 

 into colourless and coloured portions which contain granular substances, chiefly 

 starch. A nucleus, absent from the lowest forms, is clearly present in the higher; 

 the green protoplasmic substance assumes the most diverse forms, such as rings, 

 bands, or stars ; but generally, and even in the simpler forms, it breaks up into 



^ For more comprehensive works on Algse see Kiitzing. Phycologia generalis, Leipzig 1S43. — 

 Niigeli, Die neueren Algensysteme, Neuenbuig 1847. — De Bary, Bericht iiber die Fortschritte der 

 Algenkunde in den Jahren 1855, 5^, 57, in Bot. Zeitg. 1858, Supplement, p. 55. — [Hassall, Hist. 

 Brit. Freshwater Algse, 1845.— Pritchard, Infusoria.— Rabenhorst, Flora Europxa, Algarum aquae 

 dulcis et submaruiae, 1864-68.] 



