336 LECTURE XV. 



When we were discussing the osmotic properties of the cell 

 (p. 39) we found that immersion in a 4 per cent, nitre-solution 

 causes growing cells to become smaller, the reduction in size 

 being due to the abstraction of water from the vacuole, in 

 other words, to diminished turgidity. Increase in size of the 

 cell due to growth cannot be affected in this way ; it is per- 

 manent. But though the distension due to turgidity is not 

 actual growth, yet it is only when thus distended that the 

 cell-wall is capable of growth. The mechanism of the growth 

 of the cell-wall is briefly this. Let us suppose a cell-wall to 

 be of a certain size ; it is then stretched somewhat by the 

 turgidity of the cell : this temporary distension is gradually- 

 rendered permanent by growth, that is, by actual additions to 

 the substance of the cell-wall ; this increase in size is followed 

 by further distensions, and these in turn are gradually ren- 

 dered permanent by actual growth. A growing cell-wall is 

 always on the stretch ; at the same time a continuous addi- 

 tion to its substance is taking place which enables it con- 

 stantly to increase in area, and this constitutes its growth. 



We have now to deal with the question as to the mode in 

 which the additions to the substance of the cell-wall, by 

 means of which it grows in area, are made. This is a much- 

 debated question, and there are two theories concerning it 

 which especially demand our attention. The older one, due 

 to Naegeli, is known as the theory of intussusception (p. 16). 

 Starting from the fundamental idea of the micellar structure 

 of the cell-wall (p. 32), Naegeli explains the growth of cell- 

 walls as follows : when a growing cell is turgid the cell-wall 

 is stretched and its constituent micellae are so widely sepa- 

 rated from each other as to admit of the intercalation of new 

 micellae of cellulose into the intervening watery areas ; in this 

 way the extended condition of the cell-wall is rendered per- 

 manent, and by a repetition of the process further growth 

 takes place. The more recent view, of which Strasburger is 

 the principal exponent, is entirely opposed to the preceding. 

 In the first place he rejects the micellar theory, and he con- 

 siders the existence of any mode of intussusceptive growth to 

 be extremely improbable. He attributes the growth in area^ 



