80 COMPENDIUM OF GENERAL BOTANY. 



(d) The Stem of Monocotyledons^ Dicotyledons^ and 

 Gymnospertns . 



Elementary treatises on the stem-structure of the great divisions 

 of monocotyledons and dicotyledons empliasize two distinctive 

 characters, namely, that monocotyledons in general do not essen- 

 tially grow in thickness, that their ' ' closed ' ' vascular bundles are 

 scattered through the stem as seen in cross- section ; on the other 

 hand dicotyledons grow in thickness by means of a cambium 

 and their ' ' open ' ' vascular bundles are arranged in the form of a 

 ring, as shown in cross-section. Further investigations will of 

 course reveal other differences. 



The normal monocotyledonous stem is formed differently from 

 the normal dicotyledonous stem. In the palm the young plant in 

 its earliest stages of development forms a structure of considerable 

 diameter ; upon this the stem is subsequently elongated similarly to 

 the building of a tower ; a more or less embryonic condition 

 toward the apex is not excluded. The dicotyledonous stem, for ex- 

 ample — maple, grows to a considerable length during the first year ; 

 during the second year it adds to this length and also grows in 

 thickness at the basal portion according to the mechanical require- 

 ments ; that is, it has the ability to surround the stem of the first 

 year's growth with a second annual ring; during the third year an 

 additional ring is formed around the second year's growth, and 

 so on for a number of years ; therefore the base of the tree in- 

 creases in thickness, and hence in strength, corresponding to the 

 increase in height. {Gymnosperms grow in a similar manner.) 



It is well to note the fact that the specific mechanical elements 

 of dicotyledons and of monocotyledons are differently united with 

 the conducting elements. In the latter the mechanical elements 

 and the conducting elements are either entirely separated or are 

 placed in juxtaposition. 



The comparison of the two great plant-groups suggests still 

 another thought. If one recalls the mechanical principles which 

 underlie the arrangement of mechanical elements in the firm organs, 

 and if we study a cross-section of an oak or conifer, the idea nnist 

 suggest itself that rational constructive principles from a purely 

 mechanical standpoint are wholly out of the question ; for we have 

 here, leaving out of consideration the small amount of pith, almost 



