1912] Morphology and Biology of Insect Galls 321 



but around this the blade of the leaf has become about five times as thick 

 as the normal organ. A circular ridge is thus formed that produces a 

 saucer-shaped hollow in the leaf blade. This can be clearly seen in Fig. 

 33. The cells that form this ridge are placed at right angles to the blade 

 of the leaf, in nearly the position of the palisade parenchyma. There 

 has been very little increase in the number of the cells, the accretion in 

 thickness of the blade being due principally to the lengthening of the 

 cells already present in the normal leaf. 



Wherever a vein occurs in the gall, the cells are arranged in less 

 regular rows and the individual cells are much larger and not nearly so 

 elongated in outline. Intercellular spaces are not found in any part of 

 the gall tissue. The feeding habits of the larvae are such as do not 

 necessitate the destruction of the epidermis lining the gall. 



Cecidomyia pellex O.S. 

 HostFraximis americana L. 



This gall is formed by a swelling of the blade of a leaflet on each 

 side of the midrib, the cortex of which also undergoes a proliferation that 

 merges insensibly with the mesophyll. Since the production of tissue 

 is unequal on the two sides of the leaf, a folding of the blade occurs with 

 the upper surface on the inside and the midrib at the apex. The 

 depression thus formed constitutes the larval chamber. 



Dimensions: — Along the line of che midrib 10-25 mm. 



The greater part of the gall mass is produced from the mesophyll 

 of the leaflet but a small part originates from the cortex of the midrib. 

 The epidermal cells have not been stimulated to division. It is possible 

 to determine the origin of the cellular elements from the circumstances 

 that in the gall, as in the normal leaf, the veins mark the boundary be- 

 tween the palisade and the spongy parenchyma. About two-thirds has 

 originated from the spongy parenchyma and the remainder from the 

 palisade layer. The greater amount of tissue thus produced from the 

 lower surface causes the folding of' the leaflet with the sinus of the fold 

 above. 



The cells produced from the spongy parenchyma are several times 

 larger than the normal. They constitute a tissue in which intercellular 

 air spaces are entirely lacking. On the other hand, the cells that owe 

 their origin to the palisade parenchyma, while larger than the normal 

 cells, are considerably smaller than those originat2d from the spongy 

 parenchyma. The latter with their epidermal covering constitute the 

 nutritive layer of the gall. Near the surface of this tissue, where the 

 larvae are feeding, the cells have initiated divisions; here too they show 

 signs of collapsing. 



