224 THE PHYSIOLOGY OF PLANTS 



problem which horticulturists endeavour to solve, namely, 

 the production of changes in the various organs of the flower. 

 The desired change is generally the increase of the petals in 

 size and number at the expense of the sexual organs of the 

 flower. 



All the processes which cause a doubling of flowers follow 

 one line of development, and that is a reduction of specialised 

 organs to their primitive leaf-like condition and the develop- 

 ment of new whorls of leaves in the corolla. 



As we have already stated in speaking of the retrogressive 

 changes which may take place in flowers, all the diSereut 

 organs of the flower may become transformed into green leaves. 

 If we desire, therefore, a production of foliar organs in the 

 flower, we must use the means which are favourable to the 

 growth of leaves. The latter depends firstly upon the supply 

 of food material, and secondly upon the amount of water 

 available for its requirements. If there is a plentiful supply 

 of ibod material, especially of a nitrogenous nature, the multi- 

 plication of the cells will be a very active one. If at the 

 same time the organs are continuously and richly supplied 

 with water, the young cells will also be able to grow to their 

 utmost capacity. While for the production of flowering buds, 

 it is essential to decrease the supply of water and of nitrogen- 

 ous salts, to increase the phosphates supplied to the plants, 

 and to increase the illumination, double flowers require for 

 their development copious watering, an increase in the nitro- 

 genous salts in the soil, and, according to our experience, a 

 decrease in illumination. These factors, either singly or com- 

 bined, may be observed to have taken place where a leafy 

 development of flowers has taken place naturally, i.e., without 

 the aid or intention of man. On the other hand, we often find 

 that faults in the treatment of plants may produce a leafy 

 development neither desired by the grower nor desirable. 

 The proliferation of the catkins of the Hop, which is due to 

 the elongation of their axis and to the abnormal development 

 of leaves upon the latter, is caused by excessive moisture or 

 too rich a manuring with dung, or even with Chili saltpetre. 



A change in the vegetative periods, too, has been shown 

 to produce doubling. In the case of the common Cineraria 



