THICKENINGS OF TUE 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 higher 

 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 Cucurbita pepo The 

 nn A *lo ornw/wi drawing made from specimens which, bv having lain a 

 3tc., ana u io,,g time m absolute alcohol, have allowed the produc- 



pits in the stems of tion of extremely clear sections ; q, transverse view of 



most 



i -pi sieve-like septa ; fti, sieve plate on side wall 



J " 



x, thin- 

 ner parts of the longitudinal wall ; /, the same seen in 



gams. Longitudinal section : ps, contracted protoplasmic contents (lifted 

 off at sp from the transverse septum, still in contact 



sections of the stems of a t si) ; 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 Cucurbitaceae (Pumpkin, Squash, Gourd, etc.) furnish fine 



examples of sieve cells and collenchyma. 



(b) In this place may be mentioned the curious and sometimes puz- 



