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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. iQ2. Transverse section of one of the collateral mestome strands of the stem of 

 Viola tricolor arvcnis: o, portion cf cells of pericycle; e, endodermis; 1, leptome or sieve 

 cells, in among which are some collenchymatous cells (c) ; m, cambium; t, spiral tracheae or 

 vessels; g, strongly lignified tracheae; rp, medullary ray cells, the walls of which are com- 

 posed 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 



