THE STRUCTURE OF THE STEM 



357 



have no stomata. Frequently the epidermis is provided with 

 hairs of various sliapes, and occasionally secreting cells are found 

 among the others. The hairs, too, are often glandular. 



The cortex consists typically of parenchyma, with numerous 

 intercellular spaces. The outer layers frequently contain 

 chloroplasts, starch grains, &c. Bands, sheaths, or isolated 

 patches of sclerenehyma are often present. The innermost 

 layer constitutes a sheath round the stele, named the cndodermis 

 (fig. 742, en), which is sometimes conspicuous, but generally 



Fig. 742. Collateral bundle of Ricifias. en. Endodermis. ph. Phloem. 

 c. Cambium, px. Protoxylem. x. Xylem. if.c. Interfascicular cambium. 

 After Sachs. 



very difficult to identify. It can often be recognised by its cells 

 being crowded with starch grains. 



The cortex often increases along longitudinal lines, making 

 the stem ribbed, or in isolated spots, producing emergences or 

 2>ricMes. Both ribs and prickles are covered by epidermis. 



The epidermis and cortex persist throughout the life of 

 herbaceous stems, but in those which increase much in thick- 

 ness both are ultimately replaced by bark. 



Not far from the apex of the stele the procambium bundles 

 become dififerentiated, and in transverse section are seen to be 



