TISSUE SYSTEMS 



341 



water or gases. At certain places in both stems and roots 

 special structures are developed to allow of the admission of air 

 to the tissues underlying it. These are lenticeh. In stems they 

 are generally developed under places in the epidermis where 

 stomata are present. Each consists of a little rounded spherical 

 mass of corky cells arranged loosely together. They become 

 exposed to the air by rupture of the epidermis above them. In 

 the autumn a formation of cork takes place under them, by 

 which the communication with the exterior is out off till the 

 succeeding spring (flg. 724). 



Tntrastelar Ground Tissue. 



The intrastelar ground tissue is somewhat varied in its dis- 

 position. In a monostelio stem or root the whole stele is sur- 



Fl(!. 72B. Fig. 727. 



c 



Fiff.728. Diagram of monostelic stem of Dicotyledon, c. Cortex. &. Limit 



of stele, a. Pibro-vascular bundles, pi. Pith. m.r. Medullary rays. 



Fig. 727. Diagram of section of monostelio stem of monocotyledonous plant. 

 a. Vascular bundles, c. Cortex, h. Limit of stele, 



rounded by a layer of usually parenchymatous tissue abutting on 

 the endodermis and extending inwards for a variable distance. 

 This is known as the pericycle. In roots (fig. 725, p^) it is 

 generally composed of a single laj-er of cells ; in most stems, of 

 several layers. The cells are usually parenchymatous, but bands, 

 rings, or patches of sclerenchyina may be present. Frequently a 

 number of such thickened cells abut upon the vascular bundles. 

 In roots merismatic tissue is often developed in the peri- 

 cycle. In consequence of this, secondary formations of both 

 vascular tissue and periderm occur there. In the stem and 

 petiole glandular tissue is frequently met with. 



