86 DR. V. MARTIUS 



result can only be secured by means of a binding contract. The 

 green colour of glass becomes a breakfast to the sun, which, when 

 thirsty, dniiks up all the green and leaves the glass vacant. I 

 think with Hooker that blueish-green glass is best suited for our 

 purposes." Finally — what suits one place does not suit another ; 

 for instance, Paxtou's system of numerous ridge and furrow roofs, 

 so highly prized in England, would certainly not answer in our 

 snowy climate. 



Tiie nearest approximation to a perfect influence of the sun 

 would be effected, if it were possible in summer, and in favourable 

 weather, to remove the glass covering altogether during the day- 

 time, as is done in conservatories in southern Europe. But 

 as such an arrangement is impracticable, we must endeavour 

 to accommodate ourselves to circumstances as well as we can, 

 and take care to provide the greatest quantity of light ; but the 

 intense luminous, heating, and chemical powers of a tropical 

 sun, we must, once for all, give up the hope of imitating in our 

 latitudes. Thus situated, we will try to find out the degree at 

 which plants are really dependent upon light ; and as data for 

 this investigation must be derived chiefly from our own Flora, 

 we must apply them on the principle that, since each Floral 

 Domain, in every climate, is subject to the same laws of nature, 

 observations made in Germany may furnish us with certain 

 general rules, applicable to the productions of all other countries. 



Let us take the common fir as an example of the influence of 

 light and heat on the growth of forest trees. When raised from 

 seed in an open plain, it will grow regularly in height, and ex- 

 pand its branches in all directions, which will remain a longtime 

 in the form altogether of an upright cone. When the lowermost 

 become so long as to touch the ground, they die off, and the next 

 in succession take the place of them : and this process is repeated 

 as the tree advances in age and dimensions, so that, if no dis- 

 turbing injuries intervene, its coniform appearance remains still 

 the same, at a height of above eighty feet, as it was when it 

 measured only fifteen feet. At a certain age, proportioned probably 

 to the production of its branches, the individual becomes fertile, 

 and seeds are matured, of which, again, according to certain 

 influences, the proportion of barren ones gradually decreases. As 

 the fertile seeds meet in the immediate neighbourhood with 

 favourable conditions for their growth, a little grove springs up 

 round the parent stem, and a struggle of development takes 

 place among the crowded seedlings, in which some succumb, and 



