INTRODUCTION 



although not necessarily killed by the withdrawal of water, it remains 

 passive until the proper amount of water is supplied. Water pos- 

 sesses both a mechanical and a nutritive function. Unless saturated 

 with water, so that it assumes a semifluid condition, the protoplasm 

 cannot act ; moreover, all normal plant-cells must be in a turgid con- 

 dition in order to be active ; and finally, water is the vehicle by which 

 most of the food elements are brought into the cells. Water itself is 

 an important source of food, as it is decomposed by photo-synthesis 

 and supplies the hydrogen for the primary carbohydrates manufac- 

 tured in the green cells. 



The amount of water, of course, varies in different plants and in 

 different parts of the same plant. It is highest in submersed 

 aquatics like Algae, Pondweeds, etc., and lowest in dry, woody, desert 

 plants, and dried seeds and spores, which are especially adapted to 

 resist desiccation. 



Food of Green Plants. While animals can ingest solid food, this 

 is with rare exceptions impossible for plants, which absorb food in a 

 gaseous or liquid form. The main sources of food supply for green 

 plants are the C0 2 of the atmosphere, water, and dissolved mineral 

 constituents from the earth. The elements which are absolutely 

 essential are comparatively few, the most important being Oxygen, 

 Hydrogen, Carbon, and Nitrogen, which constitute the principal 

 part of the protoplasm and cell-walls ; while, in addition, Sulphur, 

 Phosphorus, Potassium, Calcium, and Iron are never absent from 

 normal green plants. 



Other elements which are not essential are regularly met with in 

 certain plants. Thus Grasses always show a large amount of Sili- 

 con ; Chlorine and Sodium are regularly found in salt-marsh plants ; 

 Iodine occurs in the large brown Seaweeds. 



Nutrition of Plants without Chlorophyll. Since t ne power of 

 assimilating C0 2 is confined to green plants, such forms as have 

 no chlorophyll must derive their carbon from organic sources. 

 Hence Moulds, Toadstools, and other Fungi, and many Flowering 

 plants, e.g. Indian-pipe (Monotropa), Snow-plant (Sarcodes), Beech- 

 drops (Epiphegus), etc., feed either as parasites upon living plants 

 or animals, or grow as saprophytes upon dead organic matter, or in 

 soils filled with decaying organic substances, like leaf-mould. A 

 small number of plants are still more like animals in their habits, 

 actually capturing living animals Insects or Crustaceans which 

 furnish them with nitrogenous food. Among the most familiar 

 of these carnivorous plants are the Pitcher-plants, Sundews, and 

 Venus's Flytrap. 



Respiration. All organisms must respire ; i.e. develop energy 

 through the decomposition of organic matter. This is in much the 

 greater number of cases oxidation of carbonaceous compounds with 



