35^ RELATION TO ENVIRONMENT. 



II. Organization of Plant Tissues. 



695. A tissue is a group of cells of the same kind having a 

 similar position and function. In large and bulky plants differ- 

 ent kinds of tissue are necessary, not only because the work of 

 the plant can be more economically performed by a division of 

 labor, but also cells in the interior of the mass or at a distance 

 from the source of the food could not be supplied with food and 

 air unless there were specialized channels for conducting food 

 and specialized tissue for support of the large plant body. In 

 these two ways most of the higher plants differ from the simple 

 ones. The tissues for conduction are sometimes called collec 

 lively the mestome, while tissues for mechanical support are 

 called stereome. Division of labor has gone further also so that 

 there are special tissues for absorption, assimilation, perception, 

 reproduction, and the like. The tissues of plants are usually 

 grouped into three systems: (i) The Fundamental System, 

 (2) The Fibrovascular System, (3) The Epidermal System. 

 Some of the principal tissues are as follows: 



1. THE FUNDAMENTAL SYSTEM. 



696. Parenchyma. Tissue composed of thin-walled cells which in the 

 normal state are living. Parenchyma forms the loose and spongy tissue in 

 leaves, as well as the palisade tissue (see Chapter IV); the soft tissue in the 

 cortex of root and stem (Fig. 414)^ as well as that of the pith, of the pith 

 rays or medullary rays of the stem; and is mixed in with the other elements 

 of the vascular bundle where it is spoken of as wood parenchyma and bast 

 parenchyma; and it also includes the undifferentiated tissue (meristem) in 

 the growing tips of roots and shoots; also the "intrafascicular" cambium 

 (i.e., between the bundles, some also include the cambium within the 

 bundle). 



697. Collenchyma. This is a strengthening tissue often found in the 

 cortex of certain shoots. It also is composed of living cells. The cells 

 are thickened at the angles, as in the tomato and many other herbs (fig. 

 414). 



698. Sclerenchyma, or stone-tissue. This is also a strengthening tissue 

 and consists of cells which do not taper at the ends and the walls are evenly 

 thickened, sometimes so thick that the inside (lumen) of the cell has nearly 

 disappeared. Usually such cells contain no living contents at maturity. 

 Sclerenchyma is very common in the hard parts of nuts, and underneath 



