118 THE STEM 



study of the secondary phloem has already been made in Exercise 

 XI. ' 



4. The stem of monocotyledons. The stem in many members 

 of this group is characterized by the fact that the stele consists 

 of collateral bundles which are more or less scattered through the 

 axis, i.e., they are not arranged in the form of a single "ring" as 

 in dicotyledons. As a consequence, the limits of cortex, pericycle 

 and pith are usually impossible to determine with exactness. 

 However, the recent work of Stover (1934) clearly shows that the 

 disposition of vascular bundles in the stems of grasses does not 

 conform to a single type. Indeed, some genera {Lcerzia, Agro- 

 pyron) possess a single series of bundles arranged in a cylinder 

 between the cortex and pith. In most monocotyledonous stems, 

 cambial activity is vestigial or usually absent from the vascular 

 bundles which are thus wholly primary in structure. Such 

 bundles are often termed "closed" bundles in contrast to the 

 "open" bundles of dicotyledons which possess secondary growth. 



Obtain a transverse section of a corn stem, and note under low 

 power the characteristic arrangement of the collateral vascular 

 hnndles which are more numerous near the periphery of the stem 

 than in the center. Study one of the large bundles under high 

 power, noting that the phloem (composed of sieve-tubes and com- 

 panion cells) is directed towards the periphery of the stem while 

 the xylem is nearest the center of the stem. The x.ylem consists 

 of four large primary xylem vessels (which are arranged like the 

 eyes, nose and mouth of a face) and a number of smaller tracheae ; 

 a prominent air-lacuna is usually present at the inner edge of the 

 innermost large vessel. The bundle is more or less completely 

 surrounded by fibers, a feature very commonly found in the 

 bundles of monocotyledonous stems. 



5. The vine type of stem in dicotyledons. This type of stem 

 often has its vascular system in the form of a "ring" of bundles 

 which are separated from each other by broad medullary rays 

 composed of parenchyma cells. Those radially directed sheets of 

 parenchyma cells may extend vertically tlie length of an internode 

 or more and in many cases continue to grow (by means of an 

 interfascicular cambium) at the same rate as the fascicular cam- 

 bium Avhich is increasing the size of the vasculai- bundles. 



