ALG^ 189 



natural group, though exhibiting remarkable variety in the degiree of 

 development of the sexual organs. So striking is the resemblance in 

 the mode of impregnation in the most highly developed genera of 

 Floridese, such as Callithamnion, Dudresnaya, or Corallina, to that in 

 Coleochsete, that it is scarcely possible to doubt the direct descent of 

 one from the other ; the chief difference is in the replacement of motile 

 antherozoids by poUinoids which have no active power of motion. The 

 process of fertilisation is the most intricate which occurs among Thallo- 

 phytes, and presents a remarkable forecast, so to speak, of the mode 

 afterwards elaborated in Flowering Plants, though only after a very long 

 interval, comprising the entire evolution of the Muscines and Vascular 

 Cryptogams. With the loss of motility of the male reproductive cells 

 is also correlated a corresponding loss of motility of the non-sexual 

 reproductive cells or tetraspores. In the higher families of the Florideae 

 we have also the highest development of the organs of assimilation found 

 among Thallophytes. If the view is correct that the higher Florideae 

 are derived directly from the Coleochsetaceas, it follows that we must 

 ' regard all the less highly developed families of this group as retrogres- 

 sions from the parent type ; and this view appears to offer the most 

 probable explanation of the true position of some aberrant forms. In 

 the HelminthodadiacecB and Squamariacece the degeneration is exhibited 

 solely in the less perfect development of their thallus or vegetative 

 structure. In the Lemaneacem this is accompanied also by a simpler 

 structure of the sexual organs. But here, as well as in Batrachospermum, 

 we have the first rudimentary appearance of a phenomenon resembling 

 that known as ' alternation of generations,' which plays so important a 

 part in the Vascular Cryptogams, and which may possibly indicate the 

 genesis of the Muscinese. In the Porphyracece we find a reduction of the 

 thallus to a simple filament or plate of cells, accompanied by only a rudi- 

 mentary development of both carpogones and trichogynes, and a limited 

 reversion to motility in the tetraspores. Regarding the Porphyraceae 

 as exhibiting retrogression froin the more complicated Florideae, rather 

 than as the lowest member of an ascending series, it is difficult to resist 

 the conclusion that the Ulvacem are derived from the Porphyraces by 

 further retrogression, displayed in the entire suppression of antherids and 

 oogones, and a reversion to the primitive conjugation of zoogametes. 

 In their vegetative structure the Ulvaceae differ widely from the isogamous 

 Confervoidese, with which they are usually associated, while the close 

 resemblance between Ulva and Porphyra is obvious. In the Florideae; 

 the Algae attain their highest type of development. 



By far the larger number of Alg» grow entirely immersed in water,, 

 running or stagnant, fresh, brackish, ,or salt ; , some float on the surface 



