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Structural and Physiological Botany. 
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of the thickening-layers we distinguish between a 
2 the pits or 
Sie pitted cells (Fig. 27); -retiulated cells (Fig. 28), spiral cells: Re 
2 | (Figs. 29, 3°), annular cells (Figs. 31-34), and scalar iform 
| (ladder-like) cells (Fig. 36). Pitted cells, which are furnished "09 
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~ in addition with spiral, reticulate, or annular thickeming- 
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beers, a Fic. 33-— Annular cell Fic. 34. — Annular cell Fic. 35.—Piece of a re- © ’ 
Rieer e 385 trom an Opuntia with from the balsam, Zad- ticulately pitted tra- ~~ 
hi -. rings passing over into samina hortensis, with |. cheide from the lime, © — 
et ae a spiral band. (x 400.) the primary cell-wall Tilia grandifolia. (X 
A ae bulging into a barrel- 400.) . SATS am 
er ht shape between the 
oR | rings. (x 400.) Sein. 
Oe _. layers, are also called ¢racheides (Fig. 35); they occur, for | 
example, abundantly in the yew, lime, and Viburnum, 
- The increase in thickness of the cell-wall takes a very peculiar form = 
Jr eae ‘in the epidermal cells of many Urticaceze and of some other plants. In 
the interior of specially enlarged cells, stratified and finally club-shaped | Paty 
tee >. outgrowths of the wall are formed, in which are deposited small crystals, - be 
her ik a; “scancely or not at all distinguishable as such, of calcium carbonate (Fig. 
ee a - _ 37), These clustered structures are known as cystoliths. Bodies of asimi- 
Jar nature occur, though less frequently, in the pith, as in Kerria japo- : 
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