CLASSIFICATION OF CELLS. 59 



zones, initial differentiation continues. Here the nascent tissue, 

 consisting of parenchyma cells, multiplying by successive divi- 

 sions, and also the nascent prosenchyma as it forms and while 

 still capable of further division, has been named Meristeni. 



201. Meristem, therefore, is not a kind of tissue, but the 

 nascent state or early condition of any tissue. It is developing 

 parenchyma, either multiplying as such, or differentiating into 

 elongated forms, as for instance, in cambium. 



Leaving the processes of cell-development to be considered 

 under the head of "Growth," and the disposition of cells and 

 tissues in the fabric to be described under the several organs 

 (root, stem, leaf, etc.) which they compose, the kinds of cells 

 are here to be indicated, without particular reference to their 

 arrangement in the plant. In all classifications of objects which 

 are understood to have been developed from one type, interme- 

 diate forms of almost every gradation are to be expected. It is 

 specially so with plant-cells ; and of them it should be said, once 

 for all, that the kinds which have received distinct names, with 

 or without sufficient reason, are only types, or leading modifica- 

 tions, some of a very marked, some of a quite subordinate 

 character. 1 



202. Plant-cells are to be described in this chapter under the 

 following classification : 



I. Cells of the fundamental system, or parenchyma cells, 

 permanent t}*pical cells. 



1. Parenchyma cells, strictly so called, including as modi- 



fications collench}-ma cells and sclerotic parenchyma 

 cells, or grit-cells, such as the lignified cells of seed- 

 coats and drupes, etc. 



2. Epidermal cells, and their modifications; e. g., Tri- 



chomes. 



3. Cork-cells, forming subcrous parenchyma, or cork. 



II. Cells and modified cells of the fibro- vascular system, pros- 

 enchyma in the widest sense. 

 1. Cells of prosenchyrna proper. 



a. Typical wood-cells and woody fibres, including libri- 



form cells (Sanio), and the secondary wood-cells 

 (De Bary). 



b. Vasiform wood-cells, or Tracheids. 



1 Sometimes a single cell in a uniform tissue may develop unlike its neigh- 

 bors as regards one or more of the following characters : form, size, nature of 

 cell-wall or cell-contents. Such cells are termed by Sachs, idioblasts. 



