THIOKENmGS OF THE WALL. 



39 



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 f ou nd 

 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 tlie 

 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 Cumrilla pepo The 



ofn onrl H.o rrnwrlprl drawing made from specimens whirh, by having lain a 

 etc., ana me c™™aea j^^^^ y^^ ,jj ^j,g„l^(g ^l^jgl^^l j^^^^ ^,1^^^^! ^^^^ °i,^^u^_ 



pits in the 

 most other 



stems 

 Phanero- 



of lion of Kxiremely clear sections ; g, transverse view of 

 eieve-like septa ; H, sieve plate on side wall ; x, thin- 

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

 "■ams Longitudinal section ; p$, contracted protoplasmic contents (lifted 



" . ■ J! 1 „ oS Ht sp from the transverse septum, still in contact 



sections of tlie stems ot at si) ; z, pareucliyma-cells between sieve-tubes, x 5ii0. 

 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 collfncliymii. 



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



