COLLENCHYMA 31 



The thick-walled elements of the plant are of considerable 

 mechanical importance, since they form skeletal tissues which 

 are mostly devoid of intercellular spaces. When such mechanical 

 elements occur in young growing organs, where too great a 

 rigidity is disadvantageous, they take the form of collenchyma, 

 which differs from other strengthening tissues in the fact that its 

 cells are living. Collenchyma is frequent immediately beneath 

 the epidermis in young stems (Fig. 34, s) , in the midribs of young 

 leaves (Fig. 51, M), and in the adult stems of some herbs (e.g. 

 Hogweed). In its most typical form it is characterised by 

 extreme thickening of the corners of the cells, as seen in trans- 

 verse section (Fig. 15), the thickened angles appearing bright 



FIG. 15. Collenchymatous tissue from the stems of the Burdock (Arctium) 

 (left) and Dead Nettle (Lamium) (right), ep., epidermis ; m.l., middle 

 lamella. Both*in transverse section. The cell-contents are shaded. 



and shining owing to their highly refractive character. Some- 

 times, however, collenchyma exhibits uniformly thickened walls, 

 interrupted only by pits. The thickening, unlike that of most 

 mechanical elements, is practically unaltered cellulose, which 

 possesses considerable stretching power, so that collenchyma is 

 well suited to act as a supporting tissue in young growing organs. 

 All cellulose-walls 1 exhibit this character, and, moreover, are 

 readily permeable to water. Cellulose is one of the more 

 complex carbohydrates, belonging to the series of the polysac- 

 charides, and composed of a large number of C 6 H 10 O 5 groups. 

 It is scarcely coloured by iodine solution, but is stained blue 



1 Mingled with the cellulose in the walls of many plant-cells, especially 

 those of succulent fruits (e.g. Apples, Gooseberries) and storage roots 

 (e.g. Beetroot), are so-called pectic substances which are soluble in water. 



