66 TISSUES 



for existence, every plant is forced to obtain the greatest possible 

 result with the smallest possible expenditure of material. This 

 " economic " principle may be said, figuratively speaking, to lay down 

 certain regulations for the construction and disposition of the various 

 tissues. Plastic materials, in particular, are often economised through 

 the selection of a suitable plan of construction. A notable illustration 

 is afforded by the skeletal system; for the main advantage entailed 

 by the construction of this system in accordance with approved 

 mechanical principles consists in the resulting economy of material. 



Finally, there is a principle of exposure of maximum surface which 

 finds a very extensive and varied application. Any expansion of 

 the superficial area of. the partition between two cells must necessarily 

 tend to increase the cohesion between the cells concerned ; this 

 circumstance is responsible, for example, for the very prevalent 

 " interlocking " of the walls of adjacent epidermal cells. In the 

 case of nutritive tissues, on the other hand, the development of large 

 septa facilitates diosrnotic interchange between different cells. The 

 exposure of the largest possible amount of surface is, of course, likewise 

 an advantage in the case of absorbing organs. The characteristic 

 structure of the absorbing tissues of roots in particular the presence 

 of root-hairs and of the haustoria of parasitic plants must be 

 interpreted mainly from this point of view. The same principle 

 is embodied in the construction of the photosynthetic system, 

 where it is responsible for the interpolation of the partitions or 

 flanges which provide a greater superficial area for the display of 

 chloroplasts. 



It is self-evident that none of the principles above enumerated 

 which may all lie regarded as particular aspects of a common under- 

 lying principle of efficiency represents an absolute " natural law." 

 There are a great many anatomical features which appear to be 

 incompatible with one or more of these governing principles. Thus, 

 quite apart from other disabilities which are attendant upon extreme 

 specialisation, the principle of division of labour occasionally comes 

 into conflict with that of economy of material. It may even happen 

 that different functions which have already been allocated to separate 

 tissues become associated again in a later ontogenetic phase, being 

 thereafter assigned to a single tissue. The trunks of Dracaena and 

 other arborescent Liliiflorae, for example, are provided, before the 

 commencement of secondary growth in thickness, both with fibres 

 specialised for mechanical support and with typical vessels serving 

 for water-conduction : in the bundles produced later by the secondary 

 meristem, on the contrary, both functions are performed by one and 

 the same form of tissue-element. 



