i84 ALG^ 



FOURTH SUBDIVISION. 

 ALG^. 



The degree of affinity between the small group of Characese and the 

 very large group of Algas is, as has already been mentioned, a point 

 on which the best authorities are not agreed. But, in passing from 

 one to the other, we finally cross the line which separates the Cormo- 

 phytes from the Thallophytes. From this point we have to do exclu- 

 sively with plants whose vegetative organs, are in no sense differentiated 

 into axial and appendicular, and which further contain no true vessels 

 and no woody tissue. On their lower limit there is no sharp line of 

 demarcation between the Algae and the chlorophyllous Protophyta, but 

 the consideration of Algae as a group by themselves, distinct from Fungi, 

 we regard not only as convenient, but as also most in accordance with 

 their probable affinities. 



Within the limits above mentioned, the degree of co'mplexity in the 

 structure of Algae is very various, and the different tjrpes will be best 

 described under the separate families. In their vegetative structure we 

 may recognise three types : the elaboration in the development of a 

 single cell, the loose association of cells into a family or ccenobe, and the 

 close aggregation of cells into a filament or a thallus. To the latter 

 belong all the higher families, and in some of these we see indications 

 of the various kinds of tissue found in vascular plants. The higher 

 forms, consisting of a well-developed thallus of large size, in which the 

 cells are associated with one another in all three directions, are almost 

 exclusively marine, and include the whole of the organisms popularly 

 known as seaweeds. In the larger forms the plant is attached to the 

 substraturn — a rock, stone, or other large alga — by a root-like organ of 

 attachment known as the disc. The attachment is, however, always 

 superficial, and the organ takes no part in the absorption of nourish- 

 ment for the plant. The organ may result from the repeated division of 

 a single cell, or it may be more complicated, being formed out of the 

 termination of the downward growth of cortical rows of cells. In nearly 

 all fresh-water Algae the single cell, the ccenobe, or the filament is en- 



