Ch. IV, 12] MONSTROSITIES OF STEMS 



203 



may have originated in the Pitcher Plants (page 76). Also 

 distinctive of leaves is a peculiar monstrosity called phyl- 

 lomania, propagated in a green-house variety of Begonia, 

 where the stem or petioles produce a great number of very 

 minute, but otherwise well-formed blades (Fig. 152). Here 

 the form-factors which shape the blade, whatever they are, 

 evidently have spread all over the plant. An extremely fine 

 division of the leaf blade, closely following the veins, some- 

 times occurs, and can be propa- 

 gated : and such is the origin 

 of the "laciniate" or finely 

 cut leaves of some cultivated 

 trees and shrubs. 



Not properly monstrosities, 

 though usually associated and 

 intergradient therewith, are 

 galls. Typical examples 

 occur in the bright red round 

 swellings on Oak leaves, which, 

 when opened, are found to 

 contain the larva of an insect 

 (Fig. 153). A common form 

 upon stems is the familiar 

 globular swelling of the stem 

 in Golden Rods. They are 

 formed by the plant tissues 

 after an insect has laid an egg 



therein, though we do not yet know t he precise natur e 

 of the stimulation w hich controls their development. The 

 growing insect feeds upon the leaf tissue, then makes its 

 way out and escapes. The advantage of the arrangement 

 to the insect is plain, but its meaning to the plant is still 

 problematical. Hundreds or thousands of such galls are 

 known, constant in form for the same kind of insect on the 

 same kind of plant. Some arc large, some small, some rough 

 or hairy, some smooth, some on leaves and some on stems, and 



Fig. 151. 



— Abnormal leaf of a 

 Potentilla. 



