I7 BOTANY OF THE LIVING PLANT 



It is oo the basis of the encysted state, strengthened and protected by cell-wall 

 tlM , ,,,,, 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- 

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



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

 , ical 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. 



Whether economy of material is actually as important in plants as it is 

 in the construction of bridges or ships by the engineer, such a principle 

 is undoubtedly manifested in the construction of many plants. In 

 bot h i • he less material that is used the lighter the structure will be. 



In the plant the material used has to be gained through photosynthesis. 

 Where the formation of a large vegetative system is involved it must 

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

 will lie in seeing how plants use their materials so as to be mechanically 

 effective. 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 III. 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 plasmolysed 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 



