1912] Morphology and Biology of Insect Galls 307 



composed of the ordinary vascular elements of the normal leaf that have 

 been stimulated to increased activity. There is not a special tracheary 

 system originated for the gall. In the galls the strands occupy a definite 

 position, since in the normal leaf they occupy a definite place in relation 

 to the spongy and the palisade parenchyma. Large glands are present 

 in the gall tissue, as shown in Fig. 14. A gland is found invariably asso- 

 ciated with a fibro-vascular strand and seems to have its counterpart in 

 the very small gland that runs through each vein of the normal leaf. 

 In some cases the abnormal glands have acicular trichomes projecting 

 into their cavities. 



Chermes abietis Choi. 

 Picea abies (L) Karst. 



\Picea mariana (Mill) B.S.P. 



A polythalamous gall produced by the swelling of the base of the 

 young shoots. Since the twigs are not usually killed the galls are sur- 

 mounted by a variable length of normal stem. The galls in general 

 vary from conoidal to nearly spherical in shape, but in some cases, owing 

 to the stimulation not having affected the entire circumference of the 

 stem, the gall does not extend completely around it and is consequently 

 less regular in outline. The surface of the gall is covered with the en- 

 larged bases of the aborted needles. These give a faceted appearance to 

 the gall and produce a likeness to a miniature pineapple. 



Dimensions: — Longer diameter 2-3 cm.; shorter diameter 1-2 cm. 



The gall in this case is a joint production of the cortex of the stem 

 and the bases of the leaves of the host. The cells of the epidermis, lin- 

 ing the gall cavities, in some cases have been prolonged to form very short 

 trichome-like structures. The hairs at the aperture of exit, as seen in 

 Fig. II, are composed of one or two celb. 



The resin ducts that occur in the normal cortex are found consider- 

 ably enlarged in the gall. In addition to these, there are out near the 

 gall periphery numerous smaller resin ducts, as shown near the margin 

 of Fig. 1 1 , that do not have corresponding structures in the unstimulated 

 tissues. These additional ducts pass in from the swollen bases of the 

 aborted leaves. A cross section near the base of these leaves cuts from 

 four to six resin ducts, while a normal leaf does not contain more than 

 two of these structures. 



Chermes fioccus, Patch. 

 Host Picea mariana (Mill) B.S.P. 



In this species the gall is produced by the swelling of the entire shoot. 

 In comparison with the former species, the leaves are little, if any, 

 swollen at the base but are more numerous on the gall than on an equal 



