CLASSIFICATION OF CELLS. 59 



zones, initial differentiation continues. Here the nascent tissue, 

 consisting of parenclij'ma cells, multipl3-ing by successive divi- 

 sions, and also tlie nascent proseuch^-nia as it forms and while 

 still capable of further division, has been named Meristem. 



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

 nascent state or earlj' condition of any tissue. It is developing 

 parcuchj-ma, 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 tlic 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 ever}- gradation are to be expected. It is 

 specially so witli 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 t3pes, or leading modiSca- 

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

 character.' 



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

 following classification : — 



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

 permanent t3-pical cells. 



1. Parench3ma cells, strictl}' so called, including as modi- 



fications collench3-ma cells and sclerotic parench3-ma 

 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 suborous parenchyma, or cork. 



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

 ench3-ma in the widest sense. 

 1. Cells of prosenclyma proper. 



a. Typical wood-cells and wood3' fibres, including libri- 



form cells (Sanio), and the secondar}' wood-cells 

 (De Bary). 



b. Vasiform wood-cells, or Trachcids. 



J Sometimes a single cell in a uniform tissue may develop nnlike its neigh- 

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

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



