COLLENCHYMA CELLS. 57 



CHAPTER XI. 



COLLENCHYMA CELLS. 



Collenchyma, or thick-angled tissue, is closely related to 

 ordinary parenchyma, but the cells are more elongated, often 

 five or six times longer than broad, prismatic in shape and thick- 

 ened at the angles. The thickenings are usually not lignified 

 and the cells contain protoplasm, a nucleus and more or less 

 chlorophyll. They are never found elsewhere than in close 

 proximity to the epidermis or rarely in a similar relation to 

 endodermal tissue, and one of the uses of the cells is evidently 

 that of giving strength to the epidermis. Sometimes collen- 

 chyma forms a continuous circle, as in the petiole of Begonia 

 and Grape; at other times it forms longitudinal bands, as in 

 stem of Yellow Dock and Cow-parsnip. 



The thickenings of the cell-walls in some plants are excessive 

 and strongly diminish the cell lumen ; in others they are slight. 

 Sometimes they are confined to the angles, while at others they 

 extend to a less degree to the rest of the cell-wall. 



The petioles of Burdock, Begonia or any species of Grape 

 afford good examples for the study of collenchyma. 



The petiole of Burdock is bluntly angular, and seven ribs, 

 darker in appearance than the rest of the surface, may be seen 

 running lengthwise and forming the angles. These are the ribs 

 of collenchyma cells. Make cross-sections and examine first in 

 water with low and high power. The cells occur in patches 

 just beneath the epidermis at the angles of the section. The 

 cell-walls are much thickened, especially at the corners, and are 

 strongly glistening (refractive), in this respect showing a 

 marked contrast to the neighboring larger parenchyma cells. 

 The cavities of the cells look dark by contrast with the shining 

 cell-walls. At the border between collenchyma and paren- 

 chyma tissue the first line of cells of the latter tissue have one- 

 half of their walls thickened, while the other half is thin. 



The walls of the collenchyma cells are marked with delicate 

 stratification lines. Such lines are common to thick walls 

 generally and are due to the different layers containing differ- 

 ent amounts of water, as may be proved by immersing in strong 

 alcohol, which removes all the water, when the lines will dis- 

 appear. Collenchyma is absent in most monocotyledonous 

 plants. In longitudinal section the cells present quite a differ- 

 ent appearance. The thickenings appear as long narrow bands, 

 the cells are elongated, some being twice, others five to six 

 times as long as broad, and blunt-ended. In some plants coTlen- 

 chyma tends strongly to fibrous tissue, being very long and 

 greatly thickened and taper-pointed. 



