14 Mr Bur don, The Pine-apple Gall of the Spruce: 



If the scales are removed from a winter bud before it begins to 

 develope in spring, the entire shoot for the following year is found 

 already laid down in miniature, with the rudimentary needles 

 closely arranged in a spiral around the abbreviated axis, which 

 they altogether conceal. 



A longitudinal section shews the stem of the young shoot to 

 be already differentiated into pith, primary vascular bundles and 

 cortex. The pith and cortical cells have square ends and are 

 densely filled with protoplasm which shews no signs of vacuola- 

 tion, rendering it evident that active growth has not yet set in. 



Chlorophyll is present in every part of the young shoot except 

 in the vascular bundles. 



Resin canals generally are absent at this stage, though here and 

 there indications that they are about to be formed may be found. 



In the outer cortex short chains of secretory cells or solitary 

 tannin sacs occur and may be recognised by the abundant deposit 

 of tannin or resin which they contain. 



The cells of the pith, and the mesophyll cells of the leaves 

 contain a copious reserve of a substance which I believe to be 

 tannin. My examination of this substance is not complete, but 

 in the meantime I will refer to it provisionally as tannin. 



Minute specks of starch are found in the chlorophyll corpuscles, 

 but the quantity is so small as to be negligible, and there is 

 nothing that can be regarded as a regular starch reserve. This 

 fact is important in view of the abundant store of starch which 

 makes its appearance at the very outset of gall formation. 



In the spring the insect awakens and begins to suck and 

 secrete a tuft of white wool-like wax from glandular plates on 

 the dorsal side of her body. The time when this happens varies 

 according to the weather. This year (1904) we had a late spring 

 in Cambridge, and it was not till the middle of April that I was 

 able to discern any signs of life. 



As soon as the insect begins to suck, the cells in 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 filled with large 

 vacuoles and the nuclei enlarge in about the same proportion. 

 The daughter cells repeat the process and 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, and extending 

 rapidly outwards through the cortex becomes excentric. 



