30 



BOTANY. 



zling Ijeruioid protrusions to be met with in some plants. When the 

 surrounding cells are very active, it sometimes happens that the thin 

 membrane which closes up a pit grows and is pushed through into 



FiQ. 21, 



the vessel, as at t 

 lower fig- 



the 



in 



ure (Fig. Slas), 

 where th repre- 

 sents the thicken- 

 ed portion of the 

 wall, and %ca the 

 thin portion clos- 

 ing the pits. Oc- 

 casionally many 

 such protrusioas 

 enter the vessel, as 

 in a in the upper 

 figure ; if these be- 

 come large" they 

 may entirely fill 

 up the cavity of 

 the vessel , as at 6, 

 wliere two large 

 ones from opposite 

 sides have met. 



Fio. 21a. 



Fig. 21. — Collenchyma cells of the Begonia, transverse seo 

 tion of the petiole. «, epidermis ; d, collenchyma-cells, with 

 thickened atigles, v, v ; chl, chlorophyll-bodies ; p, large cell of 

 parenchyma, x 550. — After Sachs 



Fig. 2io.— Hernioid protrusions into the pitted vessels of 

 Echmocystia lobata ; thi' upper figure magnified 250, and the 

 lower 1000.— From drawings by J. C. Arthur. 



36.— Theories as to the Mode of Thickening. The real 

 nature of the process in the growth in surface and thickness 



