CHAPTER XIII 



INTRODUCTION TO PLANT HISTOLOGY* 



In the previous chapters we have been considering types of organisms in 

 which the principal plant body is the gametophyte, but as we shall see in 

 the next chapters the bodies of the Pteridophyta and Spermatophyta are 

 sporophytes. These bodies are of great complexity, and before proceeding 

 further it is necessary for us to consider in some detail the types of cells 

 which contribute to their formation. 



The study of plant tissues and the cells which compose them is termed 

 plant histology. One important point must be made at the outset, namely 

 that the study of plant histology is more concerned with the architecture of 

 the cell in relation to the function which it performs than with its internal 

 composition, and therefore it is the study of cell walls rather than the study 

 of the whole cell. Moreover, as we shall see later, many of the types of cells 

 w^hich form tissues are really dead structures, containing no protoplasm, 

 although they still serve important functions. For this reason the amount of 

 material which is actually living and growling in a higher plant is a varying 

 and uncertain quantity, depending on the proportion of living to non-living 

 cells. This proportion is greatest in early life and it diminishes progressively 

 with age. In the embryonic stage all the body cells are living and actively 

 growing, but in the course of development the growth activity becomes very 

 soon localized, particularly at the tips of shoots and roots, where the tissues 

 retain the embryonic character throughout the whole life of the individual. 

 Plants thus present an opposite condition to that in animals, in which growth 

 is general in all organs until a specifically limited size is attained, after which 

 it ceases. In plants, on the contrary, growth is localized from an early stage, 

 but is maintained at these points more or less continuously. The size reached 

 by a plant is therefore, in favourable circumstances, simply a function of 

 its age. 



Meristem 



The embryonic tissue which forms the growing regions of the shoots 

 and roots is called meristem (w^m/o.? = divisible), because these are the sites 

 of rapid cell-multiplication. A certain proportion of the cells produced, 

 subsequently enlarge, cease to divide, and acquire thickened walls or undergo 

 other changes and are then added to the permanent tissues of the 

 plant. This change from meristematic to permanent form is called cell 

 differentiation. A certain number of residual cells, however, always retain 



* A more detailed treatment of the structure and development of tissues will be found 

 in Chapters XX, XXI and XXII. 



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