MERISTEMS 75 



immediately above the node as is the case in Grasses, Commelynaceae 

 and other Monocotyledons but may occupy the distal end (Galcojisis 

 Tetrahit and other Labiatae) or even the middle (Pilea oreophila) of 

 the internode. The principal advantage of this interpolation of zones 

 of meristematic activity between adult portions of the stem consists in 

 the fact, that the apical growing-point is thereby enabled to make an 

 early beginning with the formation of the floral region of the shoot : 

 valuable time is thus saved which would otherwise have to be devoted 

 to the production of cells sufficient for the ultimate development of the 

 entire shoot. Such a division of labour between apical and intercalary 

 meristems is, of course, particularly well suited to the requirements of 

 short-lived annuals, such as many of the Grasses, which have to flower 

 and fruit in the shortest possible time. 



In the preceding discussion reference has repeatedly been made to 

 the existence of more than one kind of meristematic tissue. The original 

 meristematic tissue, from which all the cells of a growing-point are 

 derived, may be termed the primordial meristem, because it in a sense 

 contains within itself all the future tissues of the organ in an unde- 

 veloped or primordial condition. The primordial meristem sooner or 

 later becomes differentiated into the three primary meristems of the 

 apical region, namely, the protoderm, the procambium and the fundamental 

 meristem. Here are already foreshadowed both the topographical 

 arrangement and the leading anatomical characteristics of the principal 

 permanent tissues : peripheral and central tissues are marked off from 

 one another, while the segregation of prosenchymatous strands from 

 masses of parenchyma, and the histological contrast between these two 

 classes of tissue, are clearly indicated. Not infrequently, also, the 

 disposition of the primary meristems throws some light upon the 

 phylogenetic origin of the permanent tissues to which they subsequently 

 give rise ; it does not, however, afford any information as to the future 

 physiological functions of these permanent tissues. 



Quite distinct from the primary meristematic layers are the secondary 

 meristems. Typically these originate from living permanent tissues, 

 certain layers or masses of which undergo repeated cell-division and 

 thus resume meristematic activity. In general terms, therefore, the 

 development of secondary meristems may be referred to the class of 

 phenomena comprised in the introductory chapter under the head of 

 " change of function." A typical illustration of the formation of a 

 secondary meristem is afforded by the conversion of a layer of green 

 cortical cells by tangential division into a cork-cambium or phellogen. 



The general properties of meristematic tissues may next be con- 

 sidered ; these are, of course, intimately connected with the special 

 activities of meristems, just as the histological peculiarities of permanent 



