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A TEXT-BOOK OF BOTANY. 



stages in their development, corresponding to the life periods of 

 the insects and changes in the constituents of the cells: (i) 

 When the galls are first formed and the larva is beginning to de- 

 velop, the cells of the outer zone, as well as those of the central 



FIG. 192. Transverse section of one of the collateral mestome strands of the 

 stem of Viola tricolor arvensis: o, portion of cells of pericycle: e, endodermis; 1, 

 leptome or sieve cells, in among which are some collenchymatous cells (c) ; m, cam- 

 bium; t, spiral tracheae or vessels; g, strongly lignified tracheae; rp, medullary ray 

 cells, the walls of which are composed of cellulose; rs, medullary ray cells the walls 

 of which are strongly lignified; s, strongly lignified cells separating the mestome 

 strands; c, collenchyma; p, pith. 



zone, contain numerous small starch grains. (2) When the in- 

 sect reaches the chrysalis stage the starch in the cells near the 

 middle of the galls is replaced in part by gallic acid, while the 

 cells at the center and near the periphery contain masses of tannic 



