THICKENINGS OF THE WALL. 29 



allowing the protoplasm of contiguous cells to become struc- 

 turally united. The sieve- like appearance of these modified 

 portions of the wall give to the Cells their name of sieve-cells. 



35. The collen- 

 chyma cells which 

 are frequently found 

 beneath the epider- 

 mis of the succulent 

 parts of h i g h e r 

 plants afford an- 

 other instance of 

 localized thicken- 

 ing. Here only the 

 angles of the cells 

 become thickened, 

 leaving broad por- 

 tions of the wall un- 

 modified (Fig. 21). 



(a) Examples of the 

 uniform thickening of 

 the cell-wall may be 

 obtained for study by 

 making thin sections of 

 the hard parts of many 

 nuts and seeds (Figs. 58 

 to 61) ; in many of these 

 more or less complex 

 channels may be found. 

 Bordered pits are best 

 studied in longitudinal 

 sections of the young 

 wood of the pines, firs, Fig. 20. Young sieve tubes of Ciicurbi/a pepo The 

 tf anr\ tlie rrnwrlprl drawing made from specimens whirh, bv having lain a 

 etc., and the crowded !,, time in absolute alcohol, have allowed the produc- 

 pits in the steins of tion of .xiremely clear sections; g, transverse view of 

 tl Tji tieve-like septa ; xi, sieve plate on side wall ; aj, thin- 



most other Phanero- ner parts of the loagifeMjind vva i| ; ;, the same seen in 

 o-ams Longitudinal section ; ps, contracted protoplasmic contents (lifted 

 , off at gp from the transverse septum, still in contact 



sections of the stems of a ts/) ; z, parenchyma-cells between sieve-tubes, x 550. 

 most annuals will yield -After Sachs. 



good examples of ringed, spiral, and reticulated thickening. The 

 stems of the Cucurbitacese (Pumpkin, Squash, Gourd, etc.) furnish fine 

 examples of sieve cells and collenchyma. 



(6) In. this place msiy be mentioned the curious and sometimes puz- 



