Vegetative Organs of Vascular Plants 133 



MONOCOTYLEDONOUS STEMS 



It is convenient to discuss first the monocotyledonous stem because 

 these stems reach a climax of differentiation in one growing season 

 and do not present the problems raised by the secondary growth of 

 dicotyledonous and gymnosperm stems. Maize may well be used as the 

 standard subject for the grass stem. Complete transverse pieces of 

 seedlings will show the overlapping whorls of leaves encircling the 

 stem. Nodal pieces show the axillary buds, the potential ears. From 

 the older plants use only the internodal pieces of stem, stripping away 

 the leaves. A pot-grown plant will become fairly well lignified and yet 

 be so small that a complete cross section, or at least a quarter sector, 

 can be placed on a slide. However, such plants give an inaccurate 

 picture of the number and structure of the bundles. To show the 

 well-developed and lignified bundle sheath and cortex or rind, use 

 large, field-grown plants at about the time of pollination. Cut the 

 stem into short disks, and divide each disk longitudinally. 



Young stems collected before the internodes have become exposed 

 should be killed in a mild fluid like Craf II. Mature stems must be 

 killed in FAA and pumped until they sink. The dry, air-filled pith is 

 difficult to infiltrate; it is therefore desirable to exhaust again in the 

 anhydrous stage of dehydration. The use of normal or tertiary butyl 

 alcohol permits paraffin embedding of all but the toughest stems, 

 which must be cut in celloidin (Fig. 13.5) . Transverse and longi- 

 tudinal sections of corn stem take a brilliant safranin-fast green stain. 

 The hemalum-safranin combination is the second choice. Iron 

 hematoxylin-safranin is used only if the middle lamella is to be 

 emphasized. 



Other important plants that illustrate the large grass type of stem 

 are sugar cane and sorghum. Wheat, oats, and other small grains and 

 field grasses illustrate the small hollow culm. The most easily available 

 grass rhizome is that of quack grass, Agropyron repens. The hydro- 

 phytic monocots have interesting culms and rhizomes. Species of 

 Carex having triangular stems, as well as round-stemmed species, and 

 the cat tail, Typha, should not be overlooked. These subjects can be 

 prepared by the methods outlined for maize. 



Monocot stems of the nongramineous type may be obtained from 

 several easily available plants. The trailing Zebrina grown in green- 

 houses has a soft stem that can be sectioned in parafi:in. Asparagus 



Fig.. 13.8— a. Stem of hemp. Cannabis saliva; b, stem of basswood, Tilia. 



