MEBISTEM. 105 



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

 it is sufficient to note here that thej- are va,rioush- combined in the 

 different organs, soinetinies formuig compact masses of tissues, 

 and at others running as slender and delicate isolated threads. 



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

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

 become differentiated 



into elongated forms 

 (cambium). For con- 

 venience of reference, 

 the meristem at the 

 growing-points of tlie 

 axis of the plant is 

 given special names : 

 Dermatogen, the 

 la^-er of nascent epi- 

 dermis ; Periblem, 

 the la.yer of nascent cortex just beneath the epidermis ; Plerom, 

 the C3linder or shaft of nascent fascicles. The cells from which 

 these primordial layers or masses of nascent tissues arise are 

 known as initial, cells} 



The initial cells produce primordial laj-ers or masses of tissues ; 

 by their further development the primordial layers or masses 

 give rise to the earl3' 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, especiallj- in plants 

 which are to live more than a single 3'ear, or which have a well- 

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

 structure, especially' hy the introduction of new elements. Such 

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

 structure of the organ. From the natnre 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 anatomy- of the plant. 



' Hanstein : Die Scheitelzellgruppp, ira Vegetation spunkt iler Phanerogamen, 

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



The distinction between meristem propei- and cambium is insisted on by 

 Niigell in his Beitrage (1858). 



Fig. 84. Longitudinal section through the tnidille of the root-tip of the embryo of 

 Ponteileria corilata. Tlie 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 ; s, the point to which the suspcnsor was attached. 

 In other terms, cc is the plerom, ec, periblem, cp, dermatogen. (Flahault.) 



