development of foliage he may be endangering the pro- 

 duction of flowers. It is a' common practice to reduce the 

 supply of water so as to encourage the formation of flower 

 buds. Some of our common British fruit trees when grown 

 in tropical climes will develop into luxuriant trees with a 

 wealth of foliage, but lack the flowers essential for the 

 production of fruit. The practice of pruning and root 

 pruning is based on the same phenomenon, and by reducing 

 the amount of vegetative organs the formation of flower 

 buds is encouraged. Yet though there seems to be this 

 opposition between vegetative and reproductive organs the 

 latter are really dependent upon the former. For it is 

 only at the expense of the food material absorbed and 

 worked up by the roots and leaves that the flowers are 

 produced. The activity of the vegetative organs must 

 therefore always precede the formation o>f flowers. 



This is seen very clearly in the case of annuals. These 

 plants complete the whole of their life-cycle in one vegeta- 

 tive season which is usually much shorter than a year, in 

 some arid regions amounting only to a few weeks during 

 and immediately after the wet season. An annual com- 

 mences as a seedling at the beginning of the favourable 

 vegetative season, and after the production of a limited 

 number of leaves produces its flowers, which are kept sup- 

 plied with food material by the activities of roots and 

 leaves until the seeds have matured, then the whole plant 

 dies down to be replaced next season by the offspring 

 developed from its seeds. Such is the life history of 

 the mustard and cress and also that of most of our com- 

 mon weeds like chickweed, groundsel, and some of our 

 grasses. The ubiquity of our weeds is due not only to 

 their effective means of dispersal, but also to their rapid 

 growth to maturity which enables them to produce two 

 and even three generations of plants in one season. 



Slower in their development and exhibiting a more 

 marked contrast of veeetative and reproductive periods 

 are the so-called biennials. In these plants after the vege- 

 tative organs are produced they are employed throughout 

 the first summer season in manufacturing and storing a 

 large supply of food material, which is to be used in the 

 formation of flowers and seed during the second year, 

 after which the entire plant dies down having fulfilled 

 its existence and produced a vast number of offspring. 

 For the large store of food material which it has laid up 



