2 THE PHYSIOLOGY OF PLANTS 



{forcing), ov, on the contrary, when we wish to produce spring 

 flowers late in the summer. 



We are able, also, by certain methods of cultivation, to 

 change the colour of flowers, or to cause them to double by a 

 transformation of stamens into petals, or by the formation 

 of new whorls of leaves. AVe can cause hard roots to become 

 thick and succulent, as in the beets, and hard fruits to become 

 large and juicy by an increase of their softer tissues (apples 

 and pears) ; and we may therefore consider ourselves able to 

 a certain degree to modify the characters of the vegetable 

 organisms, and to partially change their normal development. 



A plant must not, therefore, be looked upon as an unchange- 

 able organism, restricted to a definite form, but as a plastic 

 organism, capable of i'urther modification in all its parts. Its 

 usual shape can be altered as if it were made of wax, and it 

 can be remodelled within certain limits. This remodelling of 

 the form of a plant is, however, only possible if the gardener 

 understands how to regulate the conditions of its life, so that 

 without damaging the whole, i.e., the life of the plant, the 

 functions of the various organs may be increased beyond the 

 normal amount or reduced below the usual limit in favour of 

 some other organ. To effect these changes, it is naturally 

 essential to possess an insight into the various processes which 

 make up the life of a plant, and to understand how they are 

 affected by various external conditions — in fact, to possess a 

 knowledge of vegetable physiology. 



But though our knowledge of vegetable physiology is by no 

 means complete, still we have obtained some insight into many 

 of the processes of plant-life, and a gardener must learn how 

 to make practical use of this knowledge. For this reason we 

 start by asking : 



§ 2. What are the functions of the various organs of a plant ? 



A plant presents itself to us as a complex organism, built 

 up of a number of chambers or cells, each of which has its 

 special and, to a certain extent, independent function. The 

 functions of all cells of the body have two common objects in 

 view, namely, the preservation of tlie individual and the pro- 



