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 difficulty 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. CxALLS of ACARINA. 



Young galls of Phytoptus quadripes (Fig. 43), P. alvuvDiis (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 graduall}' 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 cavit}'. 



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 



