458 The Living Plant 



diameter) thin, filmy, green, prothallia, lying flat on the ground 

 in wet places and strongly suggesting either Liverworts or Algae. 

 On their under sides are well-protected egg-cells fertilized by male 

 cells which swim freely through water caught under the prothal- 

 lium. Then from this fertilized egg-cell arises the familiar Fern- 

 plant ; and we have here a very perfect example of that alternation 

 of generations which is of such great botanical interest. But it is 

 evident that the Fern-plants are dependent for their fertilization 

 upon the presence of standing water, though this can be supplied 

 by a flooding during rain-storms ; and this is the reason why those 

 plants are confined for the most part to shaded or moist places. 

 As to their numbers, some three thousand five hundred different 

 kinds are known, with doubtless not a great many more to be 

 found. 



With the Fern-plants are conmionly reckoned a good many 

 others which do not belong there. Indeed, to most people, any 

 plant with finely-cut foliage is thereby made a Fern, though many 

 such plants will be found to flower at intervals. The little 

 Japanese ''Air-plant," graceful, feathery and deceptively Fern- 

 like, is in fact an animal production, the tough horny skeleton of 

 a little marine Hydroid, so naturally stained and arranged that 

 not a few people declare they have witnessed it grow ! 



We turn now to the place of the Fern-plants in our tree of 

 descent (figure 177). AH evolutionary analogy would show that 

 the Moss-plants like all other groups, gave off many branches, of 

 which one in particular was a brilliant success. It was the branch 

 which developed a vascular system permitting the ready conduc- 

 tion of water; and this freed those plants from their old ground- 

 clinging habit and opened to them the upper air for the spread 

 of great masses of foliage to the sun. Thus arose the Fern- 

 plants, the earliest trees, which spread over the moister earth as 

 its dominant vegetation, a fact which our tree represents by the 

 swinging of this branch into the main trunk, displacing the Moss- 

 plants. And they have persisted to the present in their own 



