i893. ETIOLOGY OF VEGETAL GALLS. 359 



from the attacks of parasitic and inquiline enemies. How far it is 

 effectual for this purpose will be seen in the sequel. Together, these 

 several layers constitute less than one-half the diameter of the gall, 

 the remainder being composed of loose cellular parenchyma, rich in 

 tannin, over which extends a thin epidermis, formed of flattened cells, 

 and resembling, in general aspect, that found on other parts of the 

 plant. The colour of the gall, when young, is due to the presence of 

 chlorophyl granules in the Couche ceUidaire, these being readily seen 

 through the thin transparent cortex. Later on, these epidermal cells 

 become thickened and coriaceous, and the colour of the gall changes 

 to light brown. 



Passing now from the consideration of the nut-gall to that of the 

 oak-apple — the Ponimc de Chene of Reaumur — we find, in lieu of a single 

 central cavity, a more or less numerous series of closed loculi, each 

 containing a single occupant, the whole embedded in a spongy mass 

 of cellular tissue, surrounded by a common cuticle or epidermis. Like 

 the nut-gall, the oak-apple has its origin in a bud, terminal or axillary, 

 and usurps the place of what, under normal conditions, would be a 

 stem or branch. It is among the best known of our British galls — 

 appearing early in the spring, in some cases even before the scales of 

 the leaf-buds separate or the first tender green of the year is seen. 

 In favourable seasons it is met with as early as the third week in 

 April, already a blush of crimson on its delicate cheek. About the 

 middle of May, or by " Royal Oak Day " — a time when, according to 



custom, 



" Custom which all mankind to slavery brings, 

 That dull excuse for doing silly things," 



many of our king-and-constitution-loving ancestors were wont to use 

 it in the decoration of their May-boughs — the oak-apple attains, in 

 favoured situations, its full development. In very fine specimens it 

 measures from two to two-and-a-half inches in diameter ; though in 

 size it varies considerably, and in form is less regular and globular 

 than the marble nut-gall. 



About the last week in June, or the first in July, the normal 

 insect — Cynips terminaUs of Fabricius, Tevas terminalis of Marshall — 

 makes its escape from the gall, and, after a brief but active existence 

 of four or five days, devoted to the intercourse of the sexes, sickens 

 and dies, — a result to which, so far as my experience goes, there is no 

 exception : not a solitary insect survives. 



At the time of our first investigation of this subject, the generally- 

 accepted opinion was, that the eggs of Tevas terminalis were laid in the 

 summer or autumn of the year in which the gall appeared in the 

 spring ; and this at a time when the axillary and terminal buds of 

 the summer shoots, close shut in their envelope of scales, had yet so 

 far differentiated as to show a well-marked series of embryonic leaves. 

 To this view, however, a difficulty presented itself. Our investi- 

 gations showed that, at the time the insect died, these buds had not 



