ON THK I.MrORTAXCE OF LKillT AND IIKAT To I'LANTS. 77 
happen to have grown in a dark colhir, may send out shoots covered with 
small tuhers. Such are occasionally found on the aiirial branchus of 
overcrowded plants, and they have been artificially produced by placing 
growing leafy shoots in darkness. Similarly llaniincuLiiH Ficaria, the 
lesser Celandine, when growing in damp, very shady places, bears no 
flowers, but axillary corms instead. 
ANATo:\ricAL Structuke of Leaves.— This varies immensely under 
the influence of light. Leaves that are horizontally situated are said to 
be "dorsi-ventrally " constructed, the back (under side) and front (upper 
side) being very differently composed. Thus on the upper side the cuticle 
is thicker, and there are often no stomata ; while beneath it has one to four 
or more rows of oblong, vertical, compacted, the so-called pallisade cells ; 
whereas on the under side this is replaced by loose tissue full of air- 
spaces, with a thinner cuticle and numerous stomata. 
In erect, narrow leaves, more or less equally illuminated, both sides 
become more nearly alike. Peculiar lozenge-shaped cells of the epider- 
mis of a Grass are exactly imitated in a Liliaceous plant, Pasitheca ; 
while internal peculiarities of a grass-blade occur also in a Pink ; and 
Thrift imitates Barley.* 
Now these imitative microscopical structures are evidently the result 
of the leaves growing in precisely the same manner ; and since modifica- 
tions can be traced in accordance with various degrees of illumination, it 
is safe to deduce the law that such structures are the direct outcome of 
the plant's responsiveness to light. 
The Functions of Leaves. — The two great uses of all green 
structures are to elaborate the carbonic acid gas of the atmosphere into 
organised substances, and to transpire water. These are called assimila- 
tion and transpiration, and they can only be done by means of the rays 
of light. As light, however, is composed of many rays, and the solar 
spectrum embraces many others which do not afford light to our eyes, 
the question arises as to which of the rays are most efficient in the 
execution of these functions. Numerous experiments have been made by 
growing plants under coloured glasses or behind coloured fluids, or even 
in the different bands of the spectrum itself. 
The results of the writer's experiments upon assimilation have already 
appeared in this Journal,t and I would only repeat the fact that red, 
green, and violet glasses give minima," the green only feebly, while for 
transpiration these three colours give " maxima "; in other words, while 
more moisture is transpired with red and violet light, more organic matter 
is stored up under yellow, blue, and especially clear, white light. 
* For other details the reader is referred to Origin of Plant Structures, p. 238. 
t Vol. xvi. 1893, p. 59. For the writer's experiments on transpiration see Journ. 
Linn. Soc, Botany, vol. xxii. p. 81, and vol. xxiv. p. 286. 
