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 Echznocactus 
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 allowed 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 in the case of hollies are often white in colour. This 
must be regarded as due to an interference in the down- 
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 which they pro- 
duce the pale yellow colour which always 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 soine cases in which variegation is considered 
to be a disease produced by bacterial action. The so- 
