56 TISSUES 



regions concerned with the two aspects of physiological activity, 

 probably represents one of the most fundamental applications of the 

 principle of division of labour. 



It follows without further explanation that division of labour 

 within the limits of purely vegetative activity must also be facilitated 

 by the acquisition of cellular structure ; the distribution of the various 

 nutritive functions among distinct tissue-systems, in particular, is 

 scarcely conceivable in the absence of a cellular arrangement. 



II. TISSUES. 



In all multicellular plants the individual structural units combine 

 to form complex homogeneous units of a higher order of magnitude, 

 which are known as tissues. In order that a cell-complex may rank 

 as a tissue, in the anatomico-physiological sense of the term, it is 

 necessary that its component elements should exhibit a certain degree 

 of uniformity not only in respect of the functions which they perform, 

 but also with regard to the structural features correlated with these 

 functions. A great many tissues contain a certain proportion of 

 " foreign " cells, which differ very markedly as regards both structure 

 and function from the " native " elements of the tissue. A typical 

 illustration is afforded by the frequent occurrence in thin- walled green 

 photosynthetic tissues of colourless thick-walled fibrous elements, which 

 take no part in photosynthesis or in any other metabolic process, but 

 are utilised for purely mechanical purposes. The term idioblast first 

 introduced by Sachs is applied to elements of this nature. Where all 

 the idioblasts contained in a given tissue are similar in structure and 

 subserve the same purpose, they may in a sense be regarded as 

 components of a special " diffuse " tissue. 



A. MOLE OF ORIGIN OF TISSUES. 



In all the Higher Plants, from the Bryophyta upwards, the formation 

 of tissues depends, with few exceptions, upon repeated cell-division. 

 One or more primordial mother-cells (such as a spore, an oospore, a 

 single apical cell, or several initials) give rise by successive divisions 

 not only to the diverse constituents of an individual tissue, but also to 

 all the various tissues and tissue-systems in every organ of the plant- 

 body. In many Thallophyta, also, tissues arise exclusively through 

 cell-division ; but among these lower plants a process of secondary- 

 concrescence, whereby cells or even cellular filaments or cell-masses, 

 produced by division and originally separate, subsequently become 

 united, is also of frequent occurrence. 



Typical cell-division,- as illustrated by a uninucleate vegetative 



