MERISTEM. 



105 



316. As regards the course of the bundles through the plant, 

 it is sufficient to note here that they are variously combined in the 

 different organs, sometimes forming compact masses of tissues, 

 and at others running as slender and delicate isolated threads. 



317. It has been seen in 201 that ineristem is the nascent 

 state of any tissue, and that it may multiply as such, or first 

 become differentiated 



into elongated forms 

 (cambium). For con- 

 venience of reference, 

 the meristem at the 

 growing-points of the 

 axis of the plant is 

 given special names : 

 Dermatogen, the 

 layer of nascent epi- 

 dermis ; Periblem, 

 the layer of nascent cortex just beneath the epidermis ; Plerom,, 

 the cylinder or shaft of nascent fascicles. The cells from which 

 these primordial layers or masses of nascent tissues arise are 

 known as initial cells. 1 



The initial cells produce primordial layers or masses of tissues ; 

 by their further development the primordial layers or masses 

 give rise to the early distinctive tissues of an organ. The tis- 

 sues thus early formed constitute the primary structure of the 

 plant. 



318. In the further growth of an organ, especially in plants 

 which are to live more than a single year, or which have a well- 

 defined period of rest, remarkable changes may take place in its 

 structure, especially by the introduction of new elements. Such 

 changes are known as secondary, and give rise to the secondary 

 structure of the organ. From the nature of the case, it is im- 

 possible to draw a sharp line between the primary and secondary 

 structure ; but the division is nevertheless useful in the exami- 

 nation of the minute anatonn* of the plant. 



1 Hanstein : Die Scheitelzellgrupp* 1 im Vegetationspunkt der Phanerogamen, 

 1868 ; also in Botanische Abhandlungen, 1871, p. 3. 



The distinction between meristem proper and cambium is insisted on by 

 Nageli in hi.s Beitriigc (1858). 



FIG. 84. Longitudinal section through the middle of the root-tip of the embryo of 

 Pontederia cordata. The lower initial cells produce the cap. c ; the middle, the nascent 

 cortex, ec : the upper, the nascent central cylinder, cc. The nascent epidermis, ep, of 

 the stem is continued down to the cap ; , the point to which the suspensor was attached. 

 In other terms, cc is the plerom, ec, periblem, ep, deriuatogen. (Flahault.) 



