322 Plant Physiology 



In a monocotyledon, like the Indian corn, the procambial 

 strands are rather numerous, and generally irregularly 

 disposed. The differentiation of this tissue into the 

 mature elements is also complete, so that there remains 

 no growing tissue in the bundle, and there is no further 

 growth by secondary thickening. The thickening in the 

 stem which results between the young and mature stages 

 of the corn is very largely due to an increase of the size of 

 cells already laid down at an early stage. 



The leaves of many dicotyledons are generally more 

 or less completely formed with respect to fundamental 

 tissue very shortly after they begin to unfold, although 

 there may be a subsequent growth and differentiation in 

 the veins and veinlets. On the other hand, in the case of 

 certain monocotyledons, especially, a growing zone, gen- 

 erally indefinite in extent, may be maintained for a con- 

 siderable time near the base of the leaf. A few plants, 

 such as the ferns, also elongate for a time by growth at, or 

 near, the apex. 



188. Growth of the cell. It has been abundantly 

 indicated that the growth of the organs of the plant, and 

 of the plant as a whole, are dependent upon the capacity 

 of the meristem or embryonic cells to extend or to. divide. 

 Extension is commonly associated with differentiation 

 and maturity. It may result in a great relative reduc- 

 tion of the protoplasmic content, or as previously shown, 

 it may result in protoplasmic loss, and eventually in the 

 death of the cell, the firm cell-wall alone remaining. 

 This type of cell growth, therefore, usually produces a 

 specialized tissue, and the differentiation is to some 

 extent an immediate growth response, for the extent of 



