98 PLANT LIFE 



by which they can be very greatly lengthened, 

 although they become, of course, considerably 

 thinner as the result. This tissue is often 

 called collenchyma, from the peculiarly bright 

 gelatinous appearance of the walls. It is 

 specially adapted, by its extensibility, to the 

 requirements of small and growing organs, 

 whilst its inferior value as a supporting tissue 

 is largely compensated by its advantageous 

 position in the stem. Indeed, collenchyma 

 affords a wonderful example of an accurate 

 balance of qualities possessed by a tissue 

 which is required to be carefully adjusted to 

 meet very diverse needs. For whilst the 

 function of support is its main raison d'&tre, 

 it is obvious that it must not be so strong 

 or so rigid as to materially interfere with the 

 growth in length of the organ in which it is 

 present. 



It sometimes happens that the structures 

 on which the rigidity of a stem depends have 

 to be provided in a rather different way. In 

 wheat, and most other grasses, the stem 

 continues for some time to elongate just above 

 the node or " knot." Most people know that 

 it is easy to pull the stem out from the knot, 

 and that the broken end is soft and succulent. 

 But a series of such weak joints in a stem, 

 however well the mechanical requirements 

 might be fulfilled in the intervening regions, 

 would of course be fatal to the retention of 

 an erect position. In the grass this weakness 

 is remedied by a curious arrangement of the 



