322 



METAMORPHOSIS 



structure and of development, and will select for the purpose the leaf -galls so 

 well known as occurring on the leaves of the oak, a detailed acquaintance with 

 which we owe to the splendid researches of BEIJERINCK (1882). 



The galls of Dryophanta folii are green spheres changing to red, from 

 one to three cm. in diameter, attached to the ribs on the under side of the 



Fig. 95. Part of a transverse section through the bladder-gall of Viburnum lantana \ a part of the unaltered 

 leaf is seen on the left-hand side. After KUSTER (Pathol. Pflanzenanat. Jena, 1903). 



oak leaf. Beneath the green non-stomatiferous epidermis we find certain 

 spherical cells, also green. These merge inwardly into a mesophyll with un- 

 usually large intercellular spaces, characterized by possessing abundant tannin. 

 In a cavity in the interior, situated centrally, we find in autumn the insect 

 surrounded by a shell of thick-walled parenchyma. It then gnaws for itself 

 a canal out to the exterior, and breaks through in November at the onset of colder 

 weather. All the insects hatched are female, and lay eggs without fertilization, 

 a not infrequent phenomenon in the Insecta. The insect selects a resting bud 

 at the base of the older branches to lay its eggs in ; it bores through the bud- 

 scales (Fig. 96, /) and lays its egg exactly on the apex of the growing point, 

 fastening it there with a drop of slime. From the position where the egg is 

 laid one may conclude that something more than a leaf-gall will arise. The 

 egg in springtime becomes enclosed by the growing apex, and comes 

 to lie in the middle of a mass of merismatic tissue of considerable size which 

 has arisen from the growing point and its adjacent outgrowths (III). The gall 

 then arises from the bud, and is in its full-grown state about 2 mm. thick 

 and 4-5 mm. long. Its general appearance is represented at Fig. 96, IV. We 



