174 PLANT ANATOMY. 
- other on all sides, there appear between them (especially at the 
corners) intercellular spaces, which are mostly filled with air 
(Figs. 127, 129, 151, 152, 155). 
III. Cellular Tissue. 
With the exception of the one-celled plants and plant organs 
(Saccharomyces, trichomes, Lycopodium) and some fungi and 
alge represented only by simple cellular threads, all plants con- 
sist of cellular tissue,’ that is, of cells (aggregations of cells) in 
every form of arrangement. All the cells of such tissue are 
never completely uniform, but the individual parts become dis- 
tinguished at an early period in a more or less pronounced de- 
gree. While the cell of the alge performs conjointly all the 
functions which are required of the plant, in the higher plants 
a division of the work takes place in such a manner that some 
forms of tissue undertake one task and others another. 
Through this division of work there are then produced in the 
body of the plant anatomico-physiological systems of tissue. 
Such a system of tissue is, therefore, a union of cells, complete 
within itself, and connected by their entire physiological deport- 
ment. 
This differentiation of the body of the plant, however, is not 
noticeable until the later stages of development. At the places 
of development, the growing points (apex of the stem, tip of the 
_ root), such a difference in the tissues is not yet perceptible. The 
tissue here consists rather of uniform, more or less isodiametric, 
thin-walled cells containing protoplasm, and in a state of most 
active division. Such a tissue is termed developing tissue, or 
meristem." As such it stands in opposition toall the remaining 
tissues, which are also collectively comprehended under the name 
of permanent tissue. The cambium (Fig. 138) is also such a 
developing tissue. 
_ 1 The inner juicy tissue of maturing fruits (tamarinas, juniper berries, 
oranges, stone-fruits) is resolvable into individual cells, but these are al- 
ways held together by their surroundings. . 
2 Mepifoo I divide. 
