May, 1903 .] Galls and Insects Producing Them. 
423 
CONCLUSIONS. 
1. Stem galls show less variations than any other group of 
galls, although they may be produced by insects from widely 
different orders. This is undoubtedly due to the fact that the 
various insects attack corresponding parts of the host plants. In 
proof of this fact, it will be noticed that all these insects deposit 
the egg within the tissues of the host plant and not on the surface. 
2. The galls in general show an increase of parenchyma below 
the epidermis, either a thickening of cell walls or a development 
of intercellular spaces, a flattening of the fibro-vascular bundles, 
an increase of parenchyma tissue in stelar part of stem and a 
decrease in size of same. 
Part V. Development of galls. 
A very large amount of material was collected for this paper 
and great difficult}' was experienced in getting the extremely 
young stages because of the fact that young specimens were diffi- 
cult to recognize and identify. The material was carefully killed 
in either Fleming’s solution or chromo-acetic, passed through the 
alcohols, imbedded in paraffin, sectioned on a Zimmerman micro- 
tone and stained in haematoxylin. 
The galls will be considered in the same order as in Part I of 
this series. A consideration of the leaf structure is unnecessary 
since that was considered in Part I. 
I. GALLS OF ACARINA. 
Young galls of Phytoptus quadripes (Fig. 43), P. abnormis (Fig. 
44), and P. acericola (Fig. 45) were studied, and all show the 
same developmental characters. The leaf becomes slightly pitted 
on one side (usually the lower) and a corresponding elevation is 
formed on the upper surface. This gradually enlarges until the 
more or less spherical gall is produced. In P. abnormis the 
spherical gall soon assumed an elongated form. The character- 
istic cell structure of the leaf is lost and the cells become very 
irregular in shape. The elongated character of the cells just 
beneath the outer epidermis appears at a later period of the devel- 
opment. At first the inner surface of the gall is perfectly smooth, 
but very soon masses of cells are formed and project into the 
cavity (Figs. 43 and 45). At about the same time trichomes 
begin to develop from the inner epidermis (Fig. 44) and project 
into the cavity. These trichomes grow very rapidly and almost 
fill the entire cavity. 
In the very young galls no fibro-vascular bundles are formed, 
but in the older galls small bundles of fibrous tissue are numerous. 
The first effect of the insect attack is undoubtedly to cause an 
increase in the number of cells, which is an effort on the part of 
the plant to heal the wound produced by the repeated puncturing 
