400 GENERAL PLANT PATHOLOGY 



just mentioned occur tannic substances and lignin bodies. The latter 

 are produced at corners where several cells come together as local 

 thickenings of the walls. It is improbable that this lignin is nutritive 

 in function. 



Tissues of Assimilation. — Almost all galls are characterized by the 

 almost entire absence of chlorophyll. In a few galls, if present, the 

 chloroplasts are small, twisted and feebly colored besides being extremely 

 scanty. 



Vascular Tissues. — The tissue of galls is intimately associated with 

 the vascular bundles of the host plants on which the galls occur and 

 some are actually formed from the tissue of the vascular bundles. In- 

 side the galls the vascular strands are usually delicate cords both in 

 cataplasms and prosoplasms. Where they occur inside galls, we find 

 that their individual elements resemble those of the normal bundles. 

 In a few exceptional cases, as in the galls of Andricus albopunctatus, 

 these are concentric bundles. The arrangement of the gall bundles 

 varies greatly for we find them in a circle, or they pass through the bark 

 of the gall as a delicate network. 



Tissues of Aeration. — The structure of many galls is an open porous 

 one (Fig. 163). The gall parenchyma cells in some cases are star- 

 shaped, fitting together by their projections, so that large intercellular 

 spaces are formed. Stomata and lenticels constituting pneumathodes 

 are found in galls. The stomata, however, have lost their ability to 

 close and remain, therefore, permanently open. Lenticels arepresent 

 in some cases. The stomata and parts of the epidermis disintegrate and 

 large roundish lenticels develop beneath them. Perhaps this aerating 

 tissue enables the larva to get sufficient oxygen for its metabolism. 

 Anthocyanin is present in the cells of many galls, as their red cheeks 

 abundantly testify. 



Secretions and Secretory Reservoirs. — The elements concerned with 

 secretion in the normal epidermis are present in galls in unchanged form, 

 or they are increased, richly furnishing the secretions which are asso- 

 ciated with gall formations. Less frequently new forms of secreting 

 cells and tissues are found in galls. Crystals of calcium oxalate are not 

 found usually in galls, but yet their entire absence is a rare feature. 

 In some cases, the crystals when present are associated with the stereids. 



The presence of tannic bodies has been noted previously, and it 

 seems that the tannin is found in the cells of certain gall tissues. The 



