138 BOTANY OF THE LIVING PLANT 



that large plant-bodies have been made possible. But the mechanical 

 advantage conferred by the cell-wall has been gained at the sacrifice of 

 mobility. Moreover, the mechanical framework offers an obstacle to physio- 

 logical activity, and this may sometimes be a serious difficulty. The method 

 of construction of the plant-body is thus a compromise between the need for 

 mechanical strength, and for the carrying out of the vital processes. That 

 the result is in favour of the plant is shown by the success which vegetation 

 has achieved. 



Economy of material is as important in the constf uction of the plant 

 as it is in the construction of bridges, or ships by the engineer. In 

 both cases the material is costly, while the less of it that is used the 

 lighter the structure will be. In the plant the material used has to 

 be gained laboriously through Photo-Synthesis. The end to be reached 

 is the formation of as large a vegetative system as possible. But it 

 must be mechanically strong enough to maintain its form. Our chief 

 interest will lie in seeing how plants use their material to the best 

 advantage. It will be found that the methods of its use run parallel 

 to those adopted by man to gain similar results. In plants there are 

 two distinct methods of securing mechanical resistance together with 

 economy. One is through turgor of the cells, the other is by the forma- 

 tion of specific mechanical tissues. The former plays the chief part 

 while the tissues are young, the latter is effective in the mature parts 

 of the organism. But in their action they are not distinct from one 

 another. Both may be effective in the same part, and at the same 

 time. For the dependence on turgor gradually passes over to depen- 

 dence on the specific mechanical tissues, as the shoot develops and 

 its requirements become greater. 



RIGIDITY AS. BASED ON TURGOR. 



The fact that living cells are normally turgescent has already been 

 discussed in Chapter II. The firmness and rigidity of the tense cell 

 was there compared with the condition of an inflated football, or of a 

 pneumatic tyre. The elastic cellulose wall corresponds to the outer 

 cover, and the protoplast to the bladder, or to the inner tube. The 

 withered or plas.molysed cell loses its power of mechanical resistance 

 like a punctured tyre, or a deflated football. This condition holds for 

 every normally living encysted cell while young, whether isolated as 

 in some Algae, or forming a unit of some larger structure. 



In the evolution of the higher forms from simpler organisms it 

 might appear that the simplest way of extending the plant-body 

 would be to enlarge the single cell as a non-septate sac. The best of this 



