28 PROPAGATION OF PLANTS. 



small species of Gall-gnat (Cecidomyia), the eggs are 

 deposited on or in the upper surface of the leaf, and the 

 cell-matter, moving to the point of irritation, builds up 

 a trumpet-like gall (figure 9), a quarter to one-third of 

 an inch in length. These galls, when produced on the 

 leaves of wild vines having reddish petioles or leaf 

 stalks, assume the same color, and on others they are 

 greenish white, showing that the cell-matter, when re- 

 sponding to an irritant, may not change its natural 

 chemical properties any more than it would in respond- 

 ing to a stimulant, and guided by the formative prin- 

 ciple while perfecting the growth of the plant. 



We are not to suppose that these galls are necessarily 

 injurious to plants, for while the normal conditions and 

 channels of circulation may be temporarily changed, 

 followed by a re-forming of cell-matter under new con- 

 ditions, and for the express purpose of giving food and 

 protection to an insect, still the tissues of which these 

 galls are composed are generally healthy, and are, without 

 doubt, capable of performing the regular functions of 

 assimilation,, 



CHAPTER III. 

 ORIGIN AND KINDS OF^gTOS. 



The origin of buds is a subject that has received much 

 attention from vegetable physiologists, but in pursuing 

 their investigations it is to be feared that many of them 

 have been more anxious to confirm some previously con- 

 ceived theory than to discover the truth from actual per- 

 sonal research. 



Beginning at the unit of vegetable structures, we find 

 plants composed of a single cell. In the multiplication 

 or growth of these single cells a new cell is produced 



