370 MANUAL OF BOTANY 



anastomose freely with each other, forming a number of dia- 

 phragms across the cavity. 



The pericycle or external portion of the stele is sometimes 

 lignified, and forms a hard sheath to the cylinder. Its cells 

 are chiefly fibrous, serving, with the sclerenchymatous sheaths to 

 the separate bundles, to give the necessary rigidity to the stem. 

 The cortex of the stem varies a good deal in thickness, being 

 generally thin in sub-aerial and relatively thick in subterranean 

 ones. 



No cambium being present in this type of stem, there is no 

 regular increase of thickness as in the former type. The stem 

 of the young plant is at first very slight, but as growth proceeds 

 the growing point becomes continually larger and more vigorous, 

 so that each node and internode become larger than the pre- 

 ceding ones. The young stem is thus in the form of an inverted 

 cone (fig. 7.14). After a time this continuous enlargement 

 ceases, and the upper portion of the stem is cylindrical. 



This kind of stem is not associated, as is the former type, 

 with a tap root. The primary root soon disappears, and the 

 further root system consists of its branches and a number of 

 adventitious roots developed from the lower part of the stem. 



In some Monocotyledons, e.g. Dracmna, Yucca, a regular 

 growth in thickness of the stem occurs. It is brought about by 

 the development of a secondary meristem or cambium layer 

 several cells thick, which arises towards the exterior of the 

 cylinder [fig. 755). This forms a series of bundles gradually 

 proceeding outwards. These bundles are sometimes concentric, 

 having their bast internal. 



In such stems there is a formation of periderm in the cortex, 

 which is provided with lenticels. 



Othjsr Types of Stem. 



Besides the two types of monostelic stems described, another 

 is found in some of the Vascular Cryptogams. The general 

 features of the cortex vary a good deal, stereome tissue being 

 distributed in many ways. A special feature of the stele is 

 that the development of the xylem is centripetal. The xylem 

 and phloem bundles are placed side by side, and thus alternate 

 with each other. The structure is seen best in the stem of 

 Lycopodium {fig. 756). The stele is clothed by a. pseudo-peri- 

 c>cle and surrounded by an endodermis, both of which are 

 developed from the cortex. At various distances round the ring 



