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A TEXTBOOK OF THEORETICAL BOTANY 



form an inter-connected system between the cells, by means of which air can 

 permeate all the inner tissues. Up to this point cell division still occurs but 

 afterwards gradually ceases. 



The process of change is not quite continuous. Two or three pauses in 

 the process of enlargement, associated with maxima of cell division, may 

 occur before division stops. The vacuolated but still expanding tissue 

 should be classed as the sub-meristem. 



With the entry of air into the intercellular spaces the cell wall loses some 

 water and becomes more rigid, so that further expansion is resisted. The 

 intercellular substance is also changing into the middle lamella, consisting 

 of insoluble Calcium pectate and soaps, and the cells are more firmly bound 

 together. Rapid growth ceases and the cell enters the phase of internal 

 development which changes it into one or other of the types of permanent 

 tissue cells. 



These phase relationships may be summarized thus : — 



During the above time-sequence there have also been spatial changes, 

 for the activity of the initial meristem is unceasing and each differentiating 

 cell is left further and further behind by the advance of the growing point. 

 Indeed the inseparability of the ideas of space and time is nowhere better 

 displayed than in organismal development. 



We must now return to consider the initial meristem and its behaviour. 

 The characteristics of the meristem cell have been described in Chapter XIII. 

 We may recall here that each cell is a unit of semi-liquid protoplasm, enclosed 

 in a thin and highly plastic wall ; that each cell is endowed with powers of 

 active chemical synthesis and is therefore expansible, but is held in check 

 by mutual pressure against its neighbouring cells. Growth and division 

 rates among these cells are so balanced that they are all very similar in size 

 and in appearance. Close examination shows, however, that in the initial 

 meristem the surface layer of cells and one or two layers below it form distinct 

 strata, in which all cell divisions are at right angles to the surface, i.e., anti- 

 clinal. These layers therefore grow only in area or extent, but not in thickness, 

 and each cell layer remains distinct. This region is called the tunica. It is 

 characterized by its two-dimensional growth. Below the tunica lies a mass 

 of irregular cells without definite arrangement, in which divisions occur in 

 all planes and growth is therefore three-dimensional. This is called the 



