51 



called ''mosaic disease" of Tobacco plants and of 

 Tomatoes is supposed to be due to this cause, and certain 

 experiments indicate that it is highly infectious. 



Most gardeners will have come across examples of 

 abnormal formations in flowers. Often the sepals may 

 become leafy as in Jack-in-the-Green Primroses, sometimes 

 the petals have a leaf-like appearance as in the Green Rose. 

 To the flower-lover these transformations are more curious 

 than beautiful, but to the botanist they are of considerable 

 interest as cases of reversion. For we mxust assume that 

 the various floral organs are all modifications of leaves 

 which have become adapted to the special function of 

 reproduction, and in such f oliaceous developments we may 

 see a retrogression to a more primitive type of leaf. More 

 rarely, but still occasionally, we may find stamens or pistil 

 becoming transformed into vegetative leaves; the latter is 

 often the case in the flowers of the Double Cherry. In both 

 cases sterility of the flower is caused. In other sports the 

 outermost leaves of the fiower, the calyx, may become 

 coloured and delicate in texture like the corolla. This is 

 the condition in the hose-in-hose variety of the Polyanthus. 

 More frequently it is the inner leaves of the flower, the 

 stamens, that becomie petaloid. The " doubling " of 

 fl.owers is sometimes found to take place in wild plants 

 when transplanted into garden soil. In all probability 

 the tendency to doubling is independent of the cultivation 

 of the plant, but the rich nutrition which the plant receives 

 accentuates the efiect. Certainly double varieties to be 

 kept in perfect condition require an abundant supply of 

 food, and are liable to degenerate when grown in poor soil. 

 Degenerate is perhaps not a good term to use, as the plant 

 is in a more perfect condition from the reproductive point 

 of view when the stamens and pistil are normally 

 developed and fertile than when they are transformed into 

 showy petals. In that case the flower is generally com- 

 pletely sterile, though it may sometimes retain a few 

 seiviceable stamens or a receptive seed vessel. 



l^rom the doubling of flowers such as the Rose, we ixiust 

 distinguish the doubling of the so-called " flowers " of 

 Daisies, Chrysanthemums, and other members of the Com- 

 posites. In this Family the apparent flower is really a 

 head of small flowerlets or florets closely crow^ded to- 

 gether. These are often of two kinds, showy ray florets 

 and small central clisk-florets usually of a different colour. 



