THIOKENINOS 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 coUen- 

 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 (Pig. 21). 



(a) Examples of tlie 

 uniform tliickening of 

 the cell-wall may be 

 obtained for study by 

 making tliiu 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. 



wuuu. •ji. -^■^ — -, . Kg- 20.— Young sieve UiJjeB of Cucmiita pepo The 



J 1 A A drawing maile from specimens whirh, bv having lain a 



«tc., and. tue crowaea j^^j^g ^jjjjg ,„ ^l,g„lQtg alcohol, have allowed the produc- 

 Bits in the stems of tion of sxiremely clear sections ; ?, transverse viewof 

 piiH Jii i-iio -^o «■ sieve-like septa ; n, sieve plate on side wall ; x, thin- 



most other Phanero-. jjg^ parts of the longitudinal wall ; I, the same seen in 

 <r»mH Lonffitudinal section; ps, contracted piotoplasniic contents (lifted 



gams. uougiuuLLiiin,!. ^^ ^^ ^ ^^^^ ^^^ transverse septum, still in contact 

 sections of the stems of at si) ; z, parenchyma-cells between sieve-tubes, x 630. 

 most annuals will yield -After Sachs. 



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

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

 examples of sieve cells and collenchyma. 



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



