THE ^'ARIOUS FORMS OF PLANTS 



145 



The principal factors which act on plants and which make up their 

 environment are so il, water, temperature , and light. 



The first plants were probably water-loving forms. It seems 

 likelj' that, as more land appeared on the earth's surface, plants 

 became adapted to changed conditions of life on dry land. With 

 this change in habit came a need of taldng in water, of storing it, 

 of conducting it to various parts of the organism. So it does not 

 seem unlikely that plants came to have roots, stems, and leaves, and 

 thus became adapted to their environment on dry land. We find 

 in nature that those plants or animals which are best adapted or 

 fitted to live under certain conditions are the ones which survive 

 or drive other competitors out from their immediate neighborhood. 

 Nature selected those which were best fitted to live on dry land, 

 and those plants eventually covered the earth ^\dth their progeny. 

 Eventually, the forms of life grew more and more complex until 

 at last very complicated organisms such as the flowering plants 

 came to live upon the earth. Between the flowering plant and 

 the simplest of all plants are several great plant groups which act 

 as steps in complexity of structure between the most lowly and the 

 most highly specialized plants. 

 The simplest of all these forms 



are the algse. 



■ ■- ^. 

 Algaa. — The algiB are a diverse 



collection of plants, containing 

 some of the smallest and simplest 

 as weU as some of the largest 

 plants in the world. The tiny 

 one-celled plant which lives on the 

 bark of trees is an example of the 

 former ; the giant Jjglp of the Pa- 

 cific Ocean, which attains a length 

 of over one thousand feet, of the 

 latter. The body of the algse is 

 a thallus, which may be platelike, 

 circular, ribbon-formed, thread- 

 like, or filamentous. It may even 

 be composed of a single cell. A 



large number of the alga inhabit the water, frcsli and salt. In color they 

 vary from green through the shades of blue-grcisn to yellow, brown, and 

 red. The latter colors are best seen in the seaweeds, all of which, how- 



A red seaweed, showing a finelj- divided 

 thallus body. 



HUNT. ES. BIO.- 



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