WITHERED LEAVES. 213 



and tertiary series of fractions, in each of which the same law is 

 followed, in the production of the numerators and denominators, as 

 in the primary series. The secondary series occur in the follow- 

 ing order : — 



1 1 2 _3 J 8 



3 4 7 11 18 29 

 And the tertiary series will be : — 



1 1 2^ 3^ ^ 8 



4 "5 9 14 23 37 



These fractions not only denote the number of revolutions and 

 leaves in each cycle, but also indicate the angular divergence 

 of the perpendicular series of leaves. As there are 360 degrees 

 in a circle, the fraction J will represent an angle of 180 degrees; 

 yi will equal 120 degrees (Figure 6); while 2-5ths gives 144 

 degrees of divergence, or 2-5ths of a circle, as shown in Figure 7, 

 which is a transverse projection of the diagram in Figure 5. The 

 hmits of our paper will not permit us to proceed further on the 

 subject of Phyllotaxis, and so contenting ourselves with this very 

 brief reference to a deeply interesting study, we once more take 

 a glance at the withered leaves before us, and inquire if they have 

 anything more to teach us before closing. 



We have already referred to the change of colour, as being 

 partly due to chemical and physiological action in the chlorophyll 

 and cell structure of the leaf, in the processes of growth ; but may 

 we suggest that some portion at least is caused by the influence 

 of light on the fluid sap ? It is well known that plants grown in 

 darkness become etiolated or blanched, and that living vegetation 

 turns instinctively to the light, but, perhaps, we may go a step 

 farther, and trace the formation of autumn tints to the same 

 subtle power, which thus becomes both a cause and an eflect. 

 Light may be analysed by the aid of a prism, and is then seen to 

 be composed of seven colours, familiar to us all in the rainbow 

 and in the prismatic spectrum, viz., red, orange, yellow, green, 

 blue, indigo, and violet. Of these, the red, yellow, and blue, are 

 known as primary colours, since by the admixture of these in 

 certain proportions the secondary tints are produced. Thus 

 orange is a combination of red and yellow ; green, of yellow and 

 blue ; and violet, of blue or indigo and red. Again, from these 



R 



