106 ME. E. P. STEBBING ON THE LIFE-HISTOBY OF 



g 



all — which is by then bright green externally and pyramidal or pear-shaped in form, 

 from which the European gall gets its name of Pine-apple Gall — will show a number of 

 chambers situated on either side of a central axis (cf. PI. 20. tig. 2). 



The young larvae are still a bright madder-brown in colour, with legs and antennae 

 light yellow. No indications of wings have yet made their appearance (cf. PL 20. fig. 3). 



As the gall becomes partitioned off inside distinct diamond-shaped areas make their 

 appearance on the external surface, each of which serves as a cover to the chamber 

 below it. It will be noted that the galls terminate the shoots, being sessile at the top 

 of last year's woody shoot (cf. PI. 20. fig. 1). It would almost appear therefore as it' the 

 gall was a stem-growth, and to some extent it must be so looked upon, for it is made up 

 of the young needles whose growth in length has been arrested and also of the tissues 

 which would have gone to make the new stem-growth of the year. If the centre of each 

 diamond-shaped cover be examined it will be seen that a central spot on the surface is 

 lighter-coloured than the rest and usually forms a small projection which is longer and 

 more apparent in the young gall than in the old one (cf. PI. 20. fig. 1 and PL 21. fig. I ). 

 This projection would appear to represent the tip of the swollen-up and absorbed needle. 

 It is rarely more than an eighth of an inch in length and in this totally differs from the 

 European Spruce and Larch Chermes gall, in which only the lower portions of the needles 

 and shoot of the year swell up and coalesce to form the gall, the upper part of the needles 

 projecting to some distance beyond the gall-surface and being obviously the true upper 

 portion of the needle, whilst the upper portion of the shoot continues its normal growth 

 above the gall. 



Mr. E. It. Burdon's recent investigations from a botanical point of view on the origin 

 and development of the European gall * are of such interest that I feel no apology is 

 required for reproducing them here : — 



" As soon as the insect begins to suck in the spring, the cells iu the region of the cambium where 

 the apex of the proboscis lies are forced into precocious growth. They at once increase rapidly in size 

 and undergo active division. The protoplasm becomes tilled with large vacuoles and the nuclei enlarge 

 in about the same proportion. The daughter cells repeat the process aud the swelling and growth 

 radiate outwards in every direction from this centre. 



" At first the formation proceeds symmetrically and both the pith and the cortex are invaded to an 

 equal extent. Very soon, however, a limitation is imposed on the growth of the pith-cells by the 

 lignified vessels of the protoxylem, and the cells of this region do not continue to respond so readily to 

 the stimulus. The growth then takes the line of least resistance, aud extending rapidly outwards 

 through the cortex becomes excentric. . . . 



"The net result of these various changes is that almost all previous differentiation of the stem has 

 been obliterated, and in its place a parenchymatous tissue, consisting of abnormally swollen cells with 

 extremely thin walls, has been formed. The cortex on the galled side of the stem has become two to 

 three times as thick as the cortex on the other side which is still normal, and the symmetry of the stem 

 has thus been destroyed. 



* " The Pine-apple Gall of the Spruce : a Note on the Development," Proc. Camb. Phil. Soc. vol. xiii. pt. 1 

 (1905), pp. 12-19. Vide also his observations on the subject, in his other two papers in the Journ. Econ. Biol, 

 already referred to. 



