30 



BOTANY. 



zling hernioid 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 



FIG. 21. 



the vessel, as at t 

 in the lower fig- 

 ure (Fig. 21a), 

 where th repre- 

 sents the thicken- 

 ed portion of the 

 wall, and wa the 

 thin portion clos- 

 ing the pits. Oc- 

 casionally many 

 such protrusions 

 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 b, 

 where two large 

 ones from opposite 

 sides have met. 



FIG. 21 a. 



Pig. 21. Collenchyma cells of the Begonia, transverse sec- 

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

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

 parenchyma, x 550. After Sachs. 



Fig. 21a. Hernioid protrusions into the pitted vessel* of 

 Echinocystis lobata ; the 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 



