24 



MORPHOLOGY OF THE CELL. 



Fig. 21. — A, a parenchyma-cell of the cotyledon oiPhaseoliis nuiltiflortis isolated 

 by maceration ; i i the parts of the cell-wall where it is bounded by intercellular 

 spaces ; t, t the cell-wall furnished with numerous simple pits, but not greatly 

 thickened ; the thinnest parts of the pits are drawn dark. B epidermis (c) and 

 collenchyma (c/) of the leaf-stalk of a Begonia ; the epidermis-cells are uniformly 

 thickened on the outer wall where they adjoin the collenchyma, but are thickened 

 like the collenchyma at the angles where three cells meet ; these thickenings have 

 great power of swelling; chl chlorophyll grains; / parenchyma-cell (X550). 



FIG.22.— A cell beneath the epi- 

 dermis of the underground stem 

 of Pteris aquilina, isolated by 

 boiling in a solution of potassium 

 chlorate in nitric acid ; it is more 

 strongly thickened on the left side ; 

 the unthickened places here ap- 

 pear as branched channels (xsso). 



J 





'--r'l-i-^' 



Figs. 23, 24.— Young sieve-cells oiCjicurbita Pepo (X5S0) ; the preparation has been taken from pieces of the stem which by having 

 lain for a long time in absolute alcohol have allowed thel production of extremely clear sections. The sieve-plates do not at 

 present show anything of the subsequent more complicated structure, which may be examined in Nageli, /. c. ; the opening 

 of the sieve-pores has not yet begun; they are as shown in Fig. 24, sp, still closed, their contents not yet combined. Fig. 23. 

 Transverse section ; c c cambium ; / parenchyma ; si the septa of the sieve-cells, developing into sieve-plates. Fig. 24. Longi- 

 tudinal section ; g the transverse view of the sieve-like septa ; si a sieve-plate on the side-wall ; x thinner parts of the longi- 

 tudinal wall, seen at / in longitudinal section ; in them are subsequently formed a number of small sieve-pores, at present they 

 are still homogeneous ; ps the contracted protoplasm-sac, lifted off at sp from the septum ; z parenchyma-cells between the 

 sieve-cells. 



