TISSUES AND SIMPLE OEGANS. 87 



and in part tliin- walled tissue. In both monocotyledons and dicoty- 

 ledons (see figures) an imaginary line drawn through the stem in 

 cross-section first cuts the outer bundle-elements of the albumen- 

 conducting tissue, next the two secondary vessels and the wood- 

 parenchyma, and finally, still more internally, the primordial vessels 

 and the thin- walled wood -parenchyma. In both plant-divisions the 

 primordial vessels are narrower than the secondary vessels. The 

 mechanical cells of the vascular bundle of Zea Mays 

 leave four passage-ways between the fundamental 

 tissue and the interior of the bundle. The accom- 

 panying diagramatic figure (52) shows this much 

 better than the figure of the vascular bundle of Zea 

 May 8^ in which the four passages are scarcely recog- 

 nizable. They are indicated by an increase in the ^^* 



'' , '' (Diagramatic.) 



size of the cells. Since the mechanical (skeleton) 

 cells in general do not belong to the conducting bundle, we find 

 that they are not present in the hadrome of the Ricinus-\i\n\iS\e.^ 

 but are arranged in small groups near the leptome. In this 

 respect, however, the bundles of the dicotyledonous stem differ 

 very frequently. 



{e) Growth in Thickness a^tnong Dicotyledons and Monocotyledons 

 hy Means of the Cambium. 



It is important to recognize the fact that the cambium of our 

 endogenous trees and shrubs is a bipolar formative tissue ; that is, 

 each individual normal cambium- cell, when at the height of its 

 activity, must show : 1, a daughter-cell which was cut off outwardly ; 

 2, a daughter- cell which was cut off inwardly ; and 3, a cell lying 

 between the two having the power of again dividing. These 

 middle cells (3) form a cylindrical covering (cambium covering) in 

 the stems and roots. In cross-section this appears as a ring. 



Growth in thickness of stems and roots by means of the cam- 

 bium-ring produces such characteristic structural changes as will 

 astonish the young anatomist in the examination of cross-, tangential, 

 and radial sections. He will also observe numerous deviations 

 from the normal growth-type. To enter into a more particular 

 discussion of these deviations is impracticable, though it is neces- 

 sary to bear in mind that they exist. We shall now enter into a 



