74 MERISTEMATIC SYSTEM 



of preliminary differentiation. Then follows a period during which 

 both growth and differentiation reach their greatest intensity. In the 

 course of the third and final stage* the tissue acquires its full functional 

 activity. 32 It must not, however, he supposed that further histological 

 changes never take place after a tissue has attained to the " per- 

 manent " condition ; all that this term implies is, that subsequent 

 modifications do not, strictly speaking, constitute a part of the onto- 

 genetic development of the tissue. As a matter of fact, such secondary 

 alterations are usually associated with a change or loss of function. 

 Consider, for example, the case of the green cortical cells of a Beech- 

 twig, which, after acting for a long time as conducting and storing 

 elements, ultimately become thick-walled and lignified, and lose their 

 living contents : here the nature of the secondary histogenetic changes 

 shows that these cells in the end merely serve to increase the mechani- 

 cal strength of the bark, or, in other words, that they pass out of the 

 service of the nutritive into that of the protective systems. 



A general account of the meristematic tissues would be incomplete 

 if it treated them as nothing more than embryonic representatives of 

 the permanent tissues. For it must be noted that, so long as an organ 

 is undergoing growth, its embryonic tissues never become wholly con- 

 verted into permanent elements : they are, on the contrary, constantly 

 engaged in a process of self-regeneration, by virtue of which they 

 maintain an existence which is in a sense independent of their genetic 

 relation to the permanent tissues. It is from this point of view that 

 the meristematic tissues may collectively be regarded as a tissue-system 

 endowed with a definite physiological function, namely, that of pro- 

 viding the raw material employed in the construction of the permanent 

 tissues. The highly differentiated adult organs are built up entirely 

 out of the cells produced in the meristematic tissues: the most 

 obvious indication of this relation is the frequency with which cell- 

 division takes place at the growing-points. The term " meristematic 

 tissue " or " meristem " was, in fact, selected by Ni'igeli with reference 

 to this very feature. 



Hitherto only a single kind of meristematic tissue has been men- 

 tioned, namely, that which is exemplified by the apical growing-point 

 of a stem or root : while this apical meristem is indeed the most widely 

 distributed type of meristematic tissue, meristems are also found in 

 less outlying regions of the plant-body. Thus, in Grass-haulms, a zone 

 of meristematic tissue occurs just above each node : each of these zones 

 stands in the same genetic relation to the internode above it as does an 

 apical meristem to the sub-apical region of the stem. Such intercalary 

 meristems, interpolated between two regions of permanent tissue, are 

 not at all uncommon ; they are, moreover, not necessarily located 



