NATURE AND DEVELOPMENT OF PLANTS 



INTRODUCTION 



i. The Nature of the Plant. — It is a familiar fact that most 

 plants have stems, leaves and roots and that these organs are 

 variously modified. Why have such organs been developed? 

 Why do they assume such varied forms and arrangements ? How 

 does it come about that the stem reaches up in the air and that 

 the branches and leaves are arranged in a very definite order 

 while the root penetrates the soil and grows toward moisture and 

 other soil material? Is a plant, like an animal, conscious of its 

 surroundings, seeking suitable foods and avoiding unfavorable 

 conditions? It is evident to the casual observer that the plant 

 is sensitive to its surroundings ; some plant organs bend toward 

 a light that we recognize with difficulty or react to a touch that 

 we cannot appreciate. Has the plant a nervous system and 

 faculties like ourselves that enable it to adjust itself to its sur- 

 roundings? To answer these questions it will be necessary to 



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Fig. i. Diagram of a cell showing its three dimensions; mesh-work of 

 granular cytoplasm which incloses colorless cell sap, the vacuoles, and a 

 denser dark body, the nucleus. 



consider the nature of the living substance of the plant. The 

 plant body is composed of cells that are like boxes of living 

 matter having length, breadth, and thickness (Fig. i). While 

 the cells assume various forms and perform different work or 

 functions, they all possess cell walls which contain during the 



