172 PLANT LIFE. 



introduction of strengthening bands, or strings of hard 

 and interlocking fibres, at all the places where strain is 

 likely to occur. The Deadnettle or Woundwort, for 

 instance, has a very weak xylem or woodring in the 

 centre, but at the four corners of the stem there are 

 strong ribs of fibres which hold the stem erect. If 

 these ribs are carefully cut through with a sharp knife, 

 the stem has no strength, and falls over at the slightest 

 touch. In the three-cornered stem of many Sedges, 

 such as Carex and Scirpus, the strong white supporting 

 strings, placed in groups at different points near the 

 outside, are separated by green assimilating tissue, and 

 the result is exceedingly pretty, as well as of great 

 efficiency. The Rushes, and the peduncles of Umbel- 

 liferae, are also worth examining. In many cases there 

 is a central woody ring, and in the cortex, or under the 

 epidermis, stays of fibres, so that the system resembles 

 a ship's mast with supporting ratlines. This is the 

 arrangement found in Jute, Chinagrass, Hemp, and other 

 important fibre plants. The Carnation or Pink has a 

 thick ring of strengthening tissue, which has the 

 mechanical advantage of a hollow iron pillar, the 

 supporting material being as near the outside as possible 

 so as to give the greatest resistance to bending over. 



Thus, the systems of strengthening differ greatly in 

 different families of plants ; but, in every case examined, 

 it seems clear enough that the strengthening or thick- 

 ened fibres occur exactly in the position where the most 

 strain is to be encountered. 



It is almost certain that wood is a complex mixture, 

 not a simple substance ; and what little is known of its 

 formation bears out this idea. The woody thickening 

 in an ordinary tracheid, is certainly made up of layers 

 deposited one after the other, probably from the inside. 



