50 



food supply may in some cases be responsible for this 

 abnormal growth. Sometimes these forms occur spon- 

 taneously without apparent cause, and have been cultivated 

 by horticulturists, as in certain species of Echinocacttis 

 and in the flowering shoots of the Coxcomb [Celosia 

 cristata). 



Almost all cultivated plants are known to produce 

 occasionally variegated leaves, and in some cases leaves 

 which are entirely devoid of green colour. In some in- 

 stances these have been traced to lack of nutrition. We 

 know of course that if seedlings are raised in the dark or 

 potato tubers are allow^ed to sprout in the absence of light, 

 the leaves will be of a sickly yellow nature, as light is 

 essential for the production of the green colouring matter 

 characteristic of foliage. Similarly, absence of iron salts 

 in the soil will prevent the formation of chlorophyll. It 

 has also been noticed that young shoots arising below a 

 graft m the case of hollies are often white in colour. This 

 must be regarded as due to an mterference in the dow^n- 

 ward conduction of material caused by the artificial union 

 of the tissues at the graft. Some cases are also known 

 of wild plants with variegated foliage, which under culti- 

 vation in richer soil have developed normal green leaves. 

 It has also been possible by growing plants in the green- 

 house at higher temperatures to change the variegated 

 into green leaves, and to prevent the formation of further 

 variegated leaves in the case of some plants. 



Some botanists have considered that a special substance 

 or virus is developed in certain parts of a leaf which 

 has prevented the formation of the green colouring matter; 

 but as we do not know anything of the nature of such a 

 substance it is perhaps simpler to consider that the chloro- 

 phyll granules in certain portions of the leaf have re- 

 mained in a more youthful condition, in w^hich they pro- 

 duce the pale yellow colour w^hich alw^ays precedes the 

 green colouring matter. Apparently this stoppage in the 

 development occurs more commonly in plants under culti- 

 vation than in their natural condition. Variegated leaves 

 are obviously less efficient as nutritive organs, and varie- 

 gated plants are therefore often less resistant than normal 

 forms. Their foliage is more easily affected by heat or 

 frost and the leaves are less long lived. 



There are some cases in which variegation is considered 

 to be a disease produced by bacterial action. The so- 



