38 CLASSIFICATION OF CELL TYPES, TISSUES, AND TISSUE SYSTEMS 



individual cells consecutively forming many neAv cells, -which 

 because of this method of formation lie in definite rows," are 

 designated as secondary tissues. They originate from canibia 

 of various types (e.g., the vascular cambium and the cork cam- 

 bium). Tliis ontogenetic viewpoint is based on the idea that 

 since "parench^'ma" is phjdogenetically the primitive tissue, 

 meristem, which is likewise "unspecialized" and "parenchyma- 

 like," constitutes the natural foundation upon which to base a 

 classification of adult specialized tissues. This ontogenetic scheme 

 of classification is very useful in emphasizing the difference 

 between the "primary" and "secondary" growth and structure 

 of the stem and root in gymnosperms and many dicotyledons. 

 But, from the point of view of cell structure, there is often little 

 or no morphological difference between certain cell types com- 

 mon to both primary as well as secondary tissues. Thus, for 

 example, fibers which differ little in form or structure occur in 

 the cortex and pericycle ("primary tissue" regions) and in the 

 secondary phloem. 



Eames and MacDaniels further attempt to subdivide "perma- 

 nent" tissues into two main groups, viz.: (1) simple tissius, 

 such as parenchyma and collenchyma, which consist of a single 

 cell type and are thus structurally homogeneous; and (2) com- 

 plex tissues, such as xylem and i)hloem. which consist of several 

 distinct types of cells and hence are structurally heterogeneous. 

 Such a distinction appears to have a very restricted practical 

 value, although it may be theoretically justifiable on phylogenetic 

 grounds. First of all, very few of the cell types present in higher 

 vascular plants occur as "simple tissues." Parenchyma, it is 

 true, is often "homogeneous," but not infrecpiently idiohlasts, 

 in the form of branciied sclereides, are scattered among this "tis- 

 sue." From Lundegardh's standpoint, these idiobhtsts would 

 collectively compose a separate "diffuse tissue system." Further- 

 more, the elements of a "simple tissue" (e.g., fibers or par- 

 enchyma cells) may likewise be present as components of a "com- 

 j)lex tissue" (e.g., "phloem jiarenciiyma," "phloem fibers," 

 "xylem parenchyma," "xylem fibers"). 



The preceding critical resume has attempted to point out 

 briefly the advantages as well as the apparent defects of certain 



