MONOCOTYL STEM. 91 



The bundles are enclosed by a single row of cells smaller in 

 diameter than the other parenchyma cells of the section and 

 containing little or no starch. This is the endodermis or bundle 

 sheath. It is poorly developed in this type of bundle and is 

 often not present at all. It will be met with later in perfectly 

 developed form in the concentric and radial bundles. 



Note the arrangement of the whole section of this stem. It 

 represents the type of structure of all monocotyl stems. The 

 bundles are scattered without any definite order over the whole 

 cross-section, though they are more numerous in the outer part 

 than towards the center of the section. There is no true bark 

 as exists in stems of dicotyls. 



The student should be careful to differentiate between type 

 of structure and details of structure. Other monocotyl plants 

 have the same plan of arrangement as in the one just studied, 

 but perhaps none looks exactly the same. The case is very 

 much like that of human beings, all of whom are constructed 

 on the same plan, but very rarely do two tally so closely that 

 they cannot be distinguished. 



To emphasize the above point, study a cross-section of the 

 young stem of Greenbrier stained with phloroglucin. If the 

 stem be too hard to cut easily, soften it by soaking in dilute 

 alkali (1 or 2 per cent.) sufficiently long. Greenbrier is a 

 monocotyl plant of the harder or more woody kind, while 

 Spiderwort represents the soft, herbaceous plants. In the sec- 

 tion will be seen numerous bundles of the closed collateral 

 variety containing, besides vessels, both wood and bast fibres. 

 The bundles are scattered almost over the whole area of the 

 section and are imbedded in a groundwork of parenchyma cells, 

 which, however, have their wall considerably thickened, thus 

 differing from the parenchyma of Spiderwort. The area con- 

 taining the bundles is sharply divided from a narrow, outer 

 rim of the section, which resembles in a way the bark of a 

 dicotyl plant. Most monocotyl stems have such a more or less 

 distinct dividing line. In some it is a single chain of cells, in 

 others it is strongly developed; it is known as the cylinder 

 sheath. In the Greenbrier it consists of numerous hard fibrous 

 cells, which result from the crowding of incomplete bundles in 

 this region. 



Note the two very large ducts and several smaller ones in the 

 xylem or wood portion of the bundles. The xylem portion is 

 directed towards the center of the section. In the outer portion 

 next to the two large ducts is an area of softer smaller cells, 

 the phloem, made up of sieve tubes and some parenchyma cells. 

 The whole bundle is enclosed by fibres which, on the inner edge, 

 are called wood fibres, while on the outer edge they are called 

 bast fibres. 



