80 



BIOLOGY FOR BEGINNERS 



described is such a one as would be found in a dicotyledonous 

 plant. The monocotyledonous stem differs in so many ways that 



it requires special consideration. 

 Corn Stems. The common 

 corn stalk is a good example of 

 the monocotyledonous type of 

 stem. If we cut a section across 

 it, we find the tissues very dif- 

 ferently arranged from those in 

 the dicotyledonous stem, just 

 discussed. The monocotyledon, 

 in place of a bark of several lay- 

 ers, has a rind of only one kind of 

 tissue thick- walled, hard cells 

 whose function is mainly to sup- 

 port the plant. The wood, 

 cambium, and bast tissues are 

 grouped in numerous "vascular 

 bundles" which, instead of being 

 in definite rings, are scattered 

 through the stem, the larger and 

 older ones toward the center and 

 smaller and younger ones near 

 the edge. The cambium in mono- 

 cotyledons ceases to build new 

 tissue, after a time. Hence the 

 stem does not continue to in- 

 crease in diameter as does the 

 dicotyledonous stem, but pro- 

 duces tall slender plants like corn, 

 grasses, bamboos, and palm trees. 

 The bulk of the stem consists of 

 the soft thin-walled pith, instead 

 of wood and ducts, so that the 



FIG. 17. Diagram of palm stem 

 (monocot). From Sargent. 



structure is almost reversed in these two types of stems although 

 the same tissues are present. As one result of this striking dif- 



