THE SKELETON OF THE PLANT 421 



subdivisions of the trunk, ending in finely divided twigs, while 

 exposing considerable surface to the air, present on the whole 

 a fairly yielding plant body, through which the currents of air 

 can make their way without meeting such a resistance as to 

 cause the uprooting of the tree. In other forms a weak stem 

 of considerable length exists, which obtains support by clinging 

 in various ways and holding by various mechanisms to some 

 other structure, such as the trunks of trees, rocks, walls, &c. In 

 these the woody development, though considerable, is much 

 less than in ordmary trees. 



The leaves show similar adaptations of structure to environ- 

 ment. The venation of the leaves, though mainly subserving 

 the purpose of transport of liquids, is yet often such as to afford 

 considerable resistance to damage from strong winds. Often 

 the veins are so arranged as to form a series of arches on the 

 external margins, preventing lateral tearing. The epidermal 

 cells of many leaves frequently exposed to intense cold are 

 strongly cuticularised, and the rigidity thus afforded is sometimes 

 supplemented by a development of hypodermal solerenchynaa. 



AU these arrangements may be regarded as affording a 

 supporting skeleton to the plant, the nature of which and the 

 degree of its development can only be understood b}' a considera- 

 tion of the habit of hfe of the plant under examination and the 

 dangers to which its environment exposes it. 



The ordinary cell-wall of the cell as it is first formed is a 

 clear, transparent, extensible and elastic membrane, which is 

 secreted by the protoplasm and which remains in contact with 

 it so long as the cell is living. It is capable under certain con- 

 ditions of absorbing considerable qiiantities of water, and in 

 consequence swelliag considerably. It is usually said to be 

 composed of a substance named cellulose, whose chemical com- 

 position is represented by the formula n {GJi^fi-_), the value of n 

 not yet having been exactly determined. This substance is related 

 to such bodies as starch, sugar, &c., being a member of the group 

 of carbohydrates. It is capable, under the action of certain 

 reagents, of being converted into a form of sugar, and under 

 certain circumstances it can yield nutritive material for the use 

 of the living substance. 



Probably, however, the cell- wall is in very few cases alto- 

 gether composed of pure cellulose, but mixed with the latter, 

 certain bodies occur which are compounds of pectic acid with 

 lime. There are many such bodies, but our knowledge of 

 them is very imperfect, nor is their respective distribution at 



