44 GROWTH AND CELL-DIVISION 



SECTION 13. Mechanical Aids in the Differentiation of the Tissues. 



The primitive arrangement of the new cell-segments is only rarely 

 maintained permanently. It usually undergoes more or less marked 

 secondary alterations and displacement as the result of the subsequent 

 growth in the elongating zones, and still more, owing to heterogeneous 

 tissue-differentiation. The internal factors which primarily determine the 

 subsequent history of a particular segment-cell are not open to discussion 

 except from the standpoint of a problem of heredity, but we can give 

 a general outline of the mechanical ways and means utilized by the plant 

 as aids to tissue-differentiation. 



It is in all cases the inherent tendency of the different cells to 

 specific modes of growth which is primarily responsible for the differen- 

 tiation of the tissues, for it is by the varying growth- activities of different 

 cells that the tissue-strains are produced, and with them the numerous 

 direct and stimulating actions which they exercise upon the growth of 

 individual cells. 



Certain cells divide less frequently than others and hence become 

 larger, although the rate of growth may be uniform, while growth may be 

 retarded or prevented in cells attached to or surrounded by slowly growing 

 or adult tissues. A growing tissue may in fact mould itself to the space 

 available like so much plastic material, although frequently a cell or tissue 

 makes room for its own growth by penetrating between other cells, or by 

 crushing other tissues which are unable to resist the pressure brought to 

 bear upon them. Many embryo-sacs grow in this manner, and the 

 phellogen layer is able to make room for a slight formation of cork, even 

 when the stem is imbedded in a rigid plaster cast J . Similarly, the growing 

 vascular bundles may expand towards the centre of the stem and compress 

 the pith. On the other hand, an actively growing compressed tissue may 

 aid the growth of other tissues by stretching them, or may even tear them 

 asunder. 



The union between the component cells in a growing tissue is frequently 

 partially or entirely dissolved, as for example during the formation of 

 intercellular spaces, and during the separation of the abscission layers 2 in 

 fruits and leaves. This is attained by the partial or complete conversion 

 of the middle lamella of the common partition-wall into substances which 

 imbibe water, and swell or dissolve, so that a slight strain suffices to cause 

 a separation between the contiguous layers in question. Frequently the 

 weight .of the organ or the tendency of the cells to become rounded is 



1 Newcombe, Bot. Gazette, 1894, Vol. xix, p. 223. On deep-seated phellogen cf. de Bary, 

 Comp. Anat. (Clar. Press), 1884, p. 551 seq. 



3 This term is for many reasons preferable to that of ' absciss-layer.' 



