28 THE MECHANICS OF GROWTH 



tissue in the absence of oxygen l . Indeed a temporary cessation of growth 

 takes place in fungal hyphae when suddenly transferred from a concentrated 

 to a dilute solution, although the effective internal pressure may be increased 

 by 10 to 15 atmospheres. 



By means of localized softening, as well as by intussusception, 

 growth may be restricted to any particular area of the cell-wall, and hence 

 a specific shape may be assumed. It has already been shown that the 

 protoplasm is able to alter the properties of the cell-wall, and Oedogo- 

 nium affords an instance of the sudden stretching of the deposited plastic 

 cellulose without any previous increase of turgidity having occurred. 



In all cases the harmonious co-operation of various forms of energy, 

 as well as of preparatory and stimulating reactions, is necessary for the 

 continuance of growth. Hence an alteration in any one of these factors may 

 produce a cessation of growth, or a change in its character or activity. It is 

 therefore easy to see that the retardation of growth produced by a decrease 

 of turgidity does not necessarily indicate that the osmotic pressure provides 

 the mechanical energy for growth, but may be simply owing to the fact 

 that the maintenance of turgidity is one of the formal conditions for 

 growth. The action of temperature is similar in character, and beyond 

 a certain optimum a retardation and ultimate cessation of growth occur, 

 although respiration and metabolism may be more active, and although the 

 osmotic pressure may in certain cases slightly increase. A condition of 

 turgidity is also essential for secondary growth in thickness, although 

 this takes place against the osmotic pressure. In addition to its general 

 importance the latter may also, by the stretching of the cell-wall it causes, 

 aid in the deposition of new particles of cellulose between the older ones, 

 and hence play a part in growth by intussusception. This, however, re- 

 quires further proof 2 . 



Mechanical resistances exercise a direct retarding action upon growth, 

 and cause it to cease when they are sufficiently powerful, but at the same 

 time they may act as physiological stimuli. The growth of the cell-wall 

 may continue, even though external growth is impossible, and as soon as the 

 mechanical resistance is removed growth is resumed in those zones which 

 remain meristematic. The conditions are, however, not quite the same 

 when the mechanical resistance is such as to injure the tissues or to 

 disturb their normal relationships, for a special traumatic stimulus is then 

 exercised 3 . 



1 Pfeffer, Stndien z. Energetik, 1892, p. 241 ; Pflanzenphysiol., i. Aufl., Bd. II, p. 59. The fact 

 that the cell-wall is not stretched to its limit of elasticity is also shown by de Vries, Unters. ii. d. 

 mech. Ursache d. Zellstreci-ung, 1877, p. 113; Pfeffer, Drnck- u. Arbeitsleistungen, 1893, p. 404, 

 429; Schwendener and Krabbe, Jahrb. f. wiss. Bot., 1893, Bd. XXV, p. 327. 



2 Something of this kind takes place during the growth of precipitation membranes. 



s Cf. Sect. 38, Injuries. Here the formation of callus is discussed. On tyloses cf. Schellenberg, 

 Jahrb. f. wiss. Bot., 1896, Bd. xxix, p. 261 ; Mellink, Bot. Ztg., 1886, p. 749. 



