370 The Living Plant 



metrical fashion. Such disturbances of symmetry are wholly 

 natural and healthful, unlike the cases which follow. 



Disturbance of Growth by Parasitic Stimulation. — The parasites 

 of plants are of two general classes, Insects and other plants, 

 mostly of the simple kinds called Fungi. Everybody knows 

 the structures called Galls, especially common and typical upon 

 Oak leaves, where they appear as rounded, almost nut-like, often 

 hairy, sometimes red swellings which, when opened reveal al- 

 ways the presence of a living insect larva. There are hundreds 

 of kinds of these galls, very different from each other but each 

 kind so distinctive that an expert can distinguish them easily, and 

 even identify the insect which made them. Other galls are almost 

 hair-like, others are globular swellings of very slender stems, while 

 yet others include the terminal buds, and involve the leaves in 

 a way to produce those compact rose-shaped structures often 

 called willow-roses. They are all made in substantially the same 

 manner; an insect lays its egg in the growing soft tissue, and the 

 developing insect causes the plant tissue around it to form such a 

 structure, and to lay up such contents, as will provide both a safe 

 home and a sufficient food supply for the larva until its maturity, 

 when it makes its way out and away. But just how the result is 

 effected by the insect is not at all certain, whether by mechanical 

 movements or chemical secretion. Nor is it yet certain just what 

 the plant's attitude is towards the gall. It can hardly be true 

 that the plant derives benefit from the excretions of the insect, 

 since the original substance to make those excretions is mostly 

 supplied by the plant itself. It seems much more probable that 

 the plant is passively affected by the insect, which has discovered, 

 so to speak, just the chemical substance or the developmental 

 stimulus which happens so to fit some peculiarity of the metab- 

 olism of the complicated protoplasm as to stimulate it to the 

 formation of structures and substances adaptive to the uses of the 

 insect. Theoretically, man ought to be able to affect plants in 

 analogous ways, and it is not unlikely that the horticulture of the 



