310 



HUTCHINSON'S POPULAR BOTANY 



ally distinct, as in the triple flower of Fuchsia shown in fig. 377, is 

 likewise (at least, in a large number of instances) due to gall-mites. 

 Many of the singular metamorphoses of plant organs, too floral leaves 

 which are changed into foliage leaves, petals which become stamens, etc., 

 etc. are probably attributable to the same exciting cause a subject of 

 which we shall have more to say when speaking of the Flower. 



In some instances galls have a positive economic value, though this, of 



course, is of no advantage to the plant. 

 The galls of commerce are chiefly those 

 which occur on Quercus infectoria, and the 

 best of them come from Aleppo and 

 Smyrna. They yield a fine black colour 

 with any of the salts of iron, and are 

 largely used in the manufacture of writing- 

 ink. Perhaps the most dreaded of gall- 

 producing insects is the grape-louse (Phyl- 

 loxera vastatrix), which pierces with its 

 proboscis the young leaves and roots of 

 the European Vine ( Vitis vinifera), and 

 thereby causes the growths referred to. 

 The galls, by driving away nourishment 

 from the roots (in which they are ably 

 assisted by the insects themselves), starve 

 and weaken those delicate organs, and at 

 last destroy the plant. 



We have now touched upon all the 

 more important facts connected with the 

 forms, structure, and functions of foliage 

 leaves, and with the means provided by 

 Nature for their protection and preserva- 

 tion. A few remarks on the decay and fall 

 of the leaf may fitly conclude the subject. 



One of the first external signs of incipient decay in green leaves is 

 the fading of their freshness. The green becomes dull, and gradually 

 assumes a yellow, brown, or ruddy tinge, due to varying degrees of oxida- 

 tion of the chlorophyll, contained in the cells. 



The fall of the leaf is not primarily nor necessarily due to external 

 forces. Wind and frost may, and do, perform their part, but long before 

 the leaf has attained its full growth and vigour the busy protoplasts have 

 been weaving a layer of cells, which shall infallibly ensure the work of 

 disarticulation. These cells, botanically known as the layer of separation, 

 are formed in the base of the leaf-stalk, and run at right angles to the 

 older and displaced cells, so that they divide the leaf from its branch or stem. 



FIG. 377. FUCHSIA. 



Monster flower caused by fasciation. 



