352 RELATION TO ENVIRONMENT. 



What we do mean is, that where the change is not too great nor too sudden, 

 some of the plants become slightly modified. This would indicate that the 

 process of organization and change of form is a very slow one, and is there- 

 fore a question of time ages it may be in which change in environment 

 and adaptation in form and structure have gone on slowly hand in hand. 



690. Members of the plant body. The different parts into 

 which the plant body has become differentiated are from one 

 point of view, spoken of as members. It is evident that the sim- 

 plest forms of life spoken of above do not have members. It is 

 only when differentiation has reached the stage in which certain 

 more or less prominent parts perform certain functions for the 

 plant that members are recognized. In the algae and fungi 

 there is no differentiation into stem and leaf, though there is an 

 approach to it in some of the higher forms. Where this simple 

 plant body is flattened, as in the sea-wrack, or ulva, it is a jrond. 

 The Latin word for frond is thallus, and this name is applied to 

 the plant body of all the lower plants, the algae and fungi. The 

 algae and fungi together are sometimes called thallophytes, or 

 thallus plants. The word thallus is also sometimes applied to 

 the flattened body of the liverworts. In the foliose liverworts 

 and mosses there is an axis with leaflike expansions. These 

 are believed by some to represent true stems and leaves; by 

 others to represent a flattened thallus in which the margins are 

 deeply and regularly divided, or in which the expansion has only 

 taken place at regular intervals. 



In the higher plants there is usually great differentiation of 

 the plant body, though in many forms, as in the duckweeds, it is 

 in the form of a frond. While there is a great variety in the 

 form and function of the members of the plant body, they are 

 all reducible to a few fundamental members. Some reduce 

 these forms to three, the root, stem, leaf; while others to two, the 

 root, and shoot, which is perhaps the best primary subdivision, 

 and the shoot is then divided into stem and leaf, the leaf being 

 a lateral outgrowth of the stem, and can be indicated by the fol- 

 lowing diagram: 



