ELEMENTARY TISSUES 27 



The tracheids (Fig. 7) are more or less spindle-shaped short 

 cells with pitted walls and are closely fitted together. They 

 are very abundant in many plants and are the characteristic 

 woody tissues of the coniferous trees, such as the pines. 



The cell walls of wood fibers also contain, in addition to 

 cellulose, a substance known as lignin. Its chemical composition 

 is not definitely known, but it has elements and properties very 

 similar to cellulose. The fibrous tissues serve not only for the 

 strengthening of the plant but also for the conduction of certain 

 liquid and food compounds from place to place in the plant. 



Sieve Cells (Fig. 7). The sieve cells are modified fibrous 

 cells and are a part of the fibrous tissue. They are elongated 

 cells and in vertical rows. The vertical walls are thin, but the 

 transverse walls are thick and perforated with numerous open- 

 ings, permitting a free intermingling of cell contents. These 

 transverse perforated walls are known as sieve plates. The sieve 

 cells serve for the conduction of foods. 



Laticiferous Cells (Fig. 7). The laticiferous cells belong 

 to the fibrous tissue and are in simple or branching rows and are 

 sometimes modified into tubes. They are found in plants that 

 produce latex, that is, milky or sometimes colorless juices, and 

 are usually more or less closely associated with, the sieve tubes. 

 They are especially common in milkweeds, rubber plants and 

 also occur in many other plants. They contain oils, resin, muci- 

 lage, alkaloids and many other products well known to commerce. 

 They serve for the conduction of these products. 



System. A system is composed of one or more tissues and 

 serves for the performance of certain plant functions. They are 

 as follows : The fundamental system or primary meristem, the 

 epidermal, the fibro-vascular and the ground systems. 



