GENERAL MECHANICAL REQUIREMENTS OF PLANTS 151 



or horizontal position, are chiefly exposed to bonding- stresses. It' the 

 fruits of the tree are attached to long pendent stalks as in the ease 

 of the Plane the latter are subjected to longitudinal tension. In a 

 high wind, the mechanical demands are not only greater in intensity, 

 but also more varied in quality. Trunk as well as branches must then 

 withstand transverse stresses. In order, further, that the leaves may 

 not be quickly torn to shreds, the cells and tissues of which they are 

 composed must be firmly united ; in addition, the leaf-margins are, 

 as a rule, specially strengthened so as to prevent laceration. Since, 

 finally, wind tends to uproot the tree bodily, the mechanical strength 

 of the whole root-system is also tested, each portion in turn being 

 stretched taut, like the cables of a ship riding at anchor in a gale. 



It is primarily the presence of firm cellulose walls which makes it 

 possible for a plant to preserve a constant shape and to attain to out- 

 ward differentiation ; for the morphogenetic capacities of naked 

 shapeless protoplasts could not possibly become effective without the 

 aid of some resistant material such as cellulose. Similarly, it is 

 through its power of forming cell-walls, and thus acquiring the 

 requisite mechanical strength, that the plant-body and its com- 

 ponent organs are enabled to maintain a permanent specific shape. 

 Both unthickened and thickened walls are useful in this respect. 

 Thin walls do not in themselves produce any considerable degree of 

 rigidity. Their mechanical significance depends wholly upon the 

 circumstance that they can be distended by the osmotic pressure of the 

 cell-sap and so become comparatively rigid if the turgor is sufficiently 

 great. A parallel case is that of a soft thin-walled india-rubber tube, 

 which becomes comparatively hard and inflexible if air or water is 

 forced into it under pressure. The fact that herbaceous plants droop 

 on wilting, proves that thin-walled cells are capable, when turgid, of 

 sustaining the weight of the various organs which hang down in the 

 withered plant, and thus of keeping them in the positions proper to 

 their respective functions. 



It is obvious, however, that the rigidity due to the turgescence 

 of thin-walled cells is far too dependent upon external conditions, such 

 as the humidity of soil and atmosphere, to ensure the permanent 

 mechanical stability which is a vital necessity in the case of plants of 

 large size. Those conditions such as protracted high winds which 

 subject aerial organs to the severest stresses, are the very agencies that 

 also tend to diminish turgor (owing to the increase of transpiratory 

 activity which they cause), and consequently also the rigidity dependent 

 upon turgor. Hence, in order to maintain the mechanical stability 

 necessary to their welfare, large plants are forced to provide them- 

 selves with more reliable mechanical arrangements : this they can only 



