THE GROWTH OF THE CELL 267 



so we should have a Uving substance which contained no nitrogen. It is un- 

 necessary for us to trace any further the consequences of the acceptance of this 

 hypothesis ; it will be preferable to bring forward an example where we may most 

 readily consider such an active formative power in the cell-wall, viz. the bands, 

 spines, and other sculpturings formed on the surface of spores and pollen-grains 

 (WiLLE, 1886 ; Strasburger, 1889). Here we have to deal not only with a 

 simple thickening of a previously existent cell-wall, but with differential thicken- 

 ing at special places, resulting in a configuration fixed by heredity. We meet with 

 such a case in the exosporium of Selaginella galeotti (Fitting, 1900), after the 

 exosporium has been separated first from the mesosporium and later on from 

 the protoplasm. The extent to which the protoplasm has retracted renders the 

 hypothesisof migration of protoplasmintothe wall, in this case, especiallyunlikely. 



We have dwelt too long already on the question of the growth of the cell- 

 wall, although we have left unmentioned many questions which call for in- 

 vestigation, but we may refer, before leaving the subject, to one further point 

 only, the cessation of growth. If we limit ourselves to surface growth, we may 

 distinguish cells which theoretically grow on for ever, unless destroyed, from 

 those which continue to grow only for a definite time (p. 273). The question as 

 to what brings about this cessation of growth has been very variously answered. 

 Looking only at the great thickness and varying chemical characters of the 

 fully-developed cell-wall attempts have been made to give a purely mechanical 

 explanation of how such a wall has no further power of growth. As a matter of 

 fact, however, in the first beginnings of lateral branches, and also after 

 wounds, completely adult walls may start growth afresh, and we often see 

 very thick-walled cells still capable of growth (Krabbe, 1887), while thin-walled 

 cells do not show that power. The directing influence inducing or stopping growth 

 must arise from the protoplasm, for all growth phenomena are regulated by the 

 living organism. Such regulating processes meet us at every turn, wherever 

 growth and formation in plants are closely examined. It is impossible to say 

 with certainty whether definite organs in the cell, the nucleus more especially, 

 play any special part in this phenomenon. We know for certain (Townsend, 

 1897) that the protoplasmic complex can manufacture a membrane only when it 

 is provided with a nucleus, or when it is connected by protoplasmic strands — 

 however delicate — with protoplasm which contains one. It does not follow, 

 however, that cell-wall formation is a special function of the nucleus, for the 

 nucleus is no more able to form a cell-wall without protoplasm than protoplasm 

 without a nucleus. Haberlandt (1887) has pointed out that the nucleus 

 frequently approaches the region of the wall which is actively growing, and he 

 concludes from that that the nucleus has some special duties to perform in 

 relation to the formation of cellulose. At the same time cases are known in 

 which the nucleus obviously occupies other situations, and these militate against 

 the correctness of this conception. 



Reviewing the whole position we may say that the growth of the cell-wall 

 takes place in a variety of ways. All theories which involve the acceptance of 

 only one mode of growth must be rejected ; for even in those cases where growth 

 appears purely mechanical we cannot do without the complicated and invisible 

 influence of protoplasm. Protoplasm administers the stimulus which induces 

 both construction and growth of the cell-wall and it determines finally when they 

 shall cease. Protoplasm determines the degree of osmotic pressure, which must 

 decrease with every increase in the size of the cell unless the osmotically activ^e 

 material be renewed, and thus also influences the capacity for extension of the 

 cell -wall. Further, although we may speak of the cell-wall in individual cases, or 

 ingeneral,as possessing life, still that does not alter its dependence on protoplasm. 



Growth of the cell is associated frequently but not always with cell division. 

 There are plants, such as the Siphoneae and Mucorinae, which attain to quite 



