384 CHLOROPHYLL AUD LIGHT INTENSITY. 



division of labour has been so far developed, the conditions are not so simple 

 as in those plants which consist only of single cells, and it is naturally to be 

 expected that, according to the character of the individual species and the places 

 which they inhabit, the arrangements would be very varied. The fact must also 

 be kept in mind that each spot on which a plant has settled itself in the course of 

 time may undergo alterations in consequence of which the amount and strength of 

 the light affecting that part varies considerably. Long-lived plants, whicli grow 

 vigorously in height and breadth, alter in their relation to the sun in various 

 stages of growth, and must also alter their form in a corresponding manner, or, at 

 least, must alter the direction and position of their green tissues. All this requires 

 a multiplicity of contrivances which are, as a matter of fact, innumerable, and the 

 exhaustive treatment of which is scarcely possible. In order to obtain a general 

 view, it will be better to pick out some of the most remarkable of the long series 

 of arrangements whose significance lies in this, that each species of plant receives 

 for its green organs neither too much nor too little light, and to describe them in 

 their relations to light as types of smaller or larger groups. 



We will begin with those arrangements which are rendered necessary by 

 a peculiar habitat, and, first of all, we will investigate those plants which have 

 taken up their quarters in caves or grottoes, and there pass through all their 

 stages of development. In deep excavations shut off entirely from the light, aa 

 well as in those which have been formed without human interference, and those 

 which have been dug in order to obtain metal ore, coal, salt, and water, plants 

 with chlorophyU-bearing cells and tissues are completely wanting. The plants 

 which we find there consist only of pale fungi, which live on the scanty organic 

 compoxmds which the infiltrating rain-water brings with it into the depths from 

 the surface of the sunny land above, or which have established themselves on 

 organic decaying bodies brought there by chance or intentionally by animals and 

 men. It is otherwise in caves, mines, grottoes, pits, and wells, where light is able 

 to penetrate from above or from the sides, even if only through a comparatively' 

 small aperture. Truly the vegetation developed there is not very luxuriant, but 

 it is a very remarkable circumstance that there, as a rule, the plants are still green. 

 What actually astonishes one at first sight of this vegetation, flourishing in caves 

 illuminated only from one side, is the fact that they exhibit the most beautiful 

 and vigorous green, a green much fresher, indeed, and more pronounced than 

 that displayed by the plants outside. Thus the Hart's Tongue (Scolopendrium 

 offLcinarum), widely distributed in Southern Europe, when adorning the deep shady 

 walls of rocky ravines is coloured a much brighter green than when it grows on stony 

 places in the open country where light can reach it from all sides. Also the liver- 

 worts which cover the damp stones with their leaf -like thallus, in grottoes through 

 which waters ripple, show there in the half-light a distinctly richer green than when 

 outside the grotto. But this phenomenon is most striking in the prothallia of some 

 ferns belonging to the section of the Hymenophyllaceas, and in many mosses. 



A tiny moss, called popularly the Luminous Moss, but which has received from 



