COLLEXCHYMA 



■',1 



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 coUenchyma, 

 which differs from other strengthening tissues in the fact that its 

 cells are Uving. Collench3-ma 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. 

 Hogvveed). In its most tvpical 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 Xettle (Ln)Hnri)!) (right), ep., epidermis ; );!./., middle 

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



and shining owing to their highly refractive character. Some- 

 times, however, coUenchj'ma exhibits uniformly thickened waUs, 

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

 mechanical elements, is practically unaltered cellulose, which 

 possesses considerable stretching power, so that coUench\Tna is 

 well suited to act as a supporting tissue in A'oung growing organs. 

 AU cellulose-waUs ^ exhibit tliis character, and, moreover, are 

 readily permeable to water. Cellulose is one of the more 

 complex carboh3-drates, belonging to the series of the poh'sac- 

 charides, and composed of a large number of CgHioOs groups. 

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



' Jlingled 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. 



