674 REPORT — 1891. 



in lono-itudinal, not in general extensibilitj'. I presume that these writers might 

 say that the excess in longitudinal extensibility is always present whether general 

 extensibility is greater or less. In the meanwhile we must pass on to more 

 recent researches connected with surface-growth by apposition. 



In Strasburger's later work, ' Histologische Beitriige,' 1889, his views on growth 

 have undergone considerable modification. The study of certain epidermic cells, 

 of the folds in membranes, and the I'epetition of Krabbe's work on certain 

 bast fibres, have convinced him that apposition does not account for all forms of 

 o-rowth. Krabbe ' showed that in full-grown sclerenchyma {e.g. in Oleander) 

 local widenings occur without any such amount of thinning in the membrane as 

 would occur if the bulging were due to stretching. The only possible explanation 

 seems to be that there is a migration of new material into the cell-wall. Such 

 intussusception might be, as Niigeli supposed, a flow of fluid out of which new 

 micellae crystallise ; but it is now established that cellulose arises as a modification 

 of protoplasm, so that it would harmonise with our knowledge of the origin of 

 cellulose if we assume that intussusception was preceded by a wandering of proto- 

 plasm into the cell-wall. Such a state of things would render possible the 

 regulation of longitudinal growth in the case of Is^itella and Spirogyra, already 

 alluded to, as well as in growth curvatures. This view might also harmonise with 

 Wiesner's " theory that the ceU-wall contains protoplasm as long as it continues to 

 grow. 



For the sake of brevity I content myself with the above examples : I think it 

 will be allowed that there is a focussing of speculation from many sides in favour 

 of ' active ' surface-growth — or, what is perhaps a better way of putting it, in 

 favour of a belief that the extension of cell membranes depends on physiological 

 rather than physical properties, that it is in some way under the immediate 

 control of the protoplasm. We may take our choice between Wiesner's wall- 

 protoplasm (dermatoplasm), protoplasmic intussusception as conceived by Stras- 

 burger, or the action of the ectoplasm in the manner suggested by Yines,^ who 

 suppose.s that the crucial point is a change in the motility of the protoplasm, not 

 of the cell membrane. The latter theory would undoubtedly meet the difficulties — 

 if we could believe that so yielding a substance as protoplasm could resist the force 

 of turgor. 



The great difficulty is, it seems to me, that since e.g. in Oaulerpa, surface-gi-owth 

 is clearly due to stretching, as Noll has demonstrated, and since in osmotic cell-pres- 

 sure a stretching force does exist, it cannot be doubted that turgor, and ordinary 

 physical extensibility are conditions of the problem. This remains true in spite of 

 Klebs'* curious observations on the growth of plasmolysed algse, or in spite 

 of the fact that pollen tubes may grow without turgor, in spite of the same 

 being perhaps true of young cells filled with protoplasm.^ In the face of all these 

 facts, osmotic pressure in the cell must remain a vera causa tending to surface- 

 growth. 



If we accept some form of • active ' surface-growth, we must deal with turgor in 

 another way, although to do so may require a violent exercise of the imagina- 

 tion. Are we to believe, for instance, that the function of turgescence is the 

 attaining of mechanical strength? If we hold that cell-walls increase in area 

 independently of turgor, we shall be forced to invent a hypothesis such as the 

 following — which I am far from intending to uphold. It is possible to imagine 

 that the function of the force of turgor is merely to spread out the growing mem- 

 brane to its full extent, and, as it were, to make the most of it. Turgor would in 

 this respect play the part occupied by the frame used in embroidery, making it easier 

 to carry on the work satisfactorily, hut not being absolutely necessary. When 



' Pringsheim's Jahrb. xviii. - Sitz. Wien. Akad. 1886, vol. xciii. p. 17. 



' Sachs' Arheiten, 1878, and Physiology, 1886. See also Gardiner, on protoplasmic 

 contractility, in the Annals of Botany, i. p. 366. Pfeffer has, I think, shown that 

 Vines' and Gardiner's theories assume the existence of too great strength in the 

 ectoplasm. See Pfeffer in Ahliandl. der It. Sacks. Gesellsch. xvi. 1890, p. 329. 



* Tiihivjjen. Unters^ichungen, ii. p. 489. 



' See Noll, Wiirzburg. Arheiten, iii. p. 530. 



