sect, ii PHYSIOLOGY 171 



more capable it is of growth, the more it responds to the growth in its 

 neighbourhood. It forms, so to speak, the cartilaginous tissue of 

 plants. In many organs, as for example in leaf-stalks, collenchyma is 

 the permanent strengthening tissue. 



Since, as has already been pointed out, the resistance of the 

 mechanical elements to flexure is greater the farther they are removed 

 from the centre of an organ, it will be readily seen that, while a 

 flattened, outspread organ can be easily bent, if it were folded or rolled 

 together, its power of resisting a deflecting force would be increased. 

 In accordance with this principle many leaves become plaited or rolled 

 (Fig. 1 70), and so acquire a sufficient rigidity without the assistance of 

 any specially developed mechanical tissues. 



II. Nutrition 



By nutrition are understood all the processes of metabolism, or 

 the chemical transformation and conversion of matter carried on 

 by plants in the production and appropriation of their food-supply. 

 Without nourishment and without new building material no growth 

 or development is possible. As the processes of elaboration and 

 secretion are continuous, if the food-supply is not kept equal to the 

 demands made upon it, the death of the organism from starva- 

 tion must ensue, while a continuance of its growth and further 

 development is only possible when there is a surplus of the elaborated 

 food material. 



The Constituents of the Plant Body. — By means of chemical 

 analysis the constituent substances of plants have been accurately 

 ascertained. It requires, however, no analysis to realise that a part, 

 often indeed the greater part, of the weight of a plant is derived from 

 the water with which the whole plant is permeated. Water not only 

 fills the cavities of living, fully-developed cells, but it is also present in 

 the protoplasm, cell walls, and starch grains. By drying at a tempera- 

 ture of 110°-120° C. all water may be expelled from vegetable tissues, 

 and the solid matter of the plant will alone remain. The amount of 

 dried substance in plants varies according to the nature and variety of 

 the plant and of the particular organ. In woody parts it constitutes 

 up to 50 per cent of their weight, but in herbaceous plants amounts 

 to only 20 or 30 per cent. In more succulent plants and fruits it 

 makes up only 5-15 per cent of their total weight; in water-plants 

 and Algae, 2-5 per cent, while everything else is water. 



The dried substance of plants is combustible, and consists of 

 organic compounds, which contain but little oxygen, and are converted 

 by combustion into simple inorganic compounds, for the most part 

 into carbonic acid and water. The elements carbon, hydrogen, and 

 OXYGEN form the chief constituents of the combustible dried substance. 



