96 



SCIENCE. 



[N. S. Vol. XVII. No. 420 



is safe to assume that the progenitors of 

 the higher plants lived in the water. The 

 nearest approach to these ancestral forms 

 which have survived are probably certain 

 green algas, which have retained much of 

 their primitive simplicity. Much the 

 greater number of living plants, however, 

 have given up the primitive aquatic habit 

 for life on land. In adapting themselves 

 to this new habitat they have contrived to 

 exist with a much diminished water sup- 

 ply, which has enabled them to outstrip 

 the much simpler forms which have re- 

 tained their old aquatic habit. 



The change from the primitive aquatic 

 condition to the much more varied condi- 

 tions of terrestrial existence is bound up 

 with profound changes in the organization 

 of the plant body. 



MARINE PLANTS. 



Of the existing plants which have re- 

 tained the primitive aquatic habit, the 

 most important are the various types of 

 marine algre, including not only the larger 

 seaweeds, but also the minute pelagic forms 

 like the diatoms and Peridine^. Many of 

 the larger seaweeds are very much better 

 developed than the simple green fresh- ^ 

 water algse, and show many special modi- 

 fications associated with their peculiar en- 

 vironment. Not being subject to the dry- 

 ing up which threatens all fresh-water 

 organisms at times, it is very rarely that 

 marine algffi develop any form of resting- 

 spores such as are so common among fresh- 

 water algae. On the other hand, those 

 which grow between tide-marks, where 

 they are regularly exposed at low-tide, de- 

 velop mucilaginous or gelatinous tissues, 

 which prevent too complete loss of water. 

 This is especially well seen in the large 

 kelps and similar forms. Some of these, 

 also, reach an enormous size, and develop 

 leaves which are often provided with 

 bladder-like floats, which bring them to 



the surface when they are exposed to the 

 light. 



Very characteristic are the minute pe- 

 lagic plants, especially the diatoms and 

 Peridineas, which are important constitu- 

 ents of the plankton, or surface life of the 

 sea. These floating plants are generally 

 provided with some sort of buoyant appa- 

 ratus, evidently an adaptation to their 

 pelagic life. Small as these floating algse 

 are individually, they are immensely im- 

 portant to ocean life, as they constitute 

 the main source of food for the hosts of 

 animals inhabiting the sea. 



The great subkingdom of fungi offers 

 many interesting problems bearing upon 

 the evolution of plant-forms, but there is 

 no reason to suppose that any higher types 

 of plants have ever arisen from the fungi, 

 many of which are doubtless plants of 

 comparatively recent origin. Most of 

 their peculiarities are associated with their 

 nutrition, which is entirely different from 

 that of typical plants. Not having chloro- 

 phyll, they are, like animals, dependent 

 upon other organisms for food. Conse- 

 quently all fungi are either saprophytes, 

 living upon dead organic matter, or as 

 parasites they attack living animals and 

 plants. 



I can not dwell here upon the extremely 

 difficult problems connected with the origin 

 and affinities of the fungi, even if I felt 

 competent to discuss them. 



THE ORIGIN OF TERRESTRIAL PLANTS. 



We have now to consider what causes 

 led to the abandonment of the aquatic 

 habit by the alga ancestors of the vascular 

 plants, and how this radical change in 

 their environment has influenced the de- 

 velopment of the structures of the higher 

 plants. 



Nearly all fresh-water plants are ex- 

 posed to destruction at times, by the dry- 

 ing up of the bodies of water in which 



