Plant World Continued 



229 



Fig. 125. A. — Diagrammatic Cross-section of Stem of Indian Corn (endogenous or 

 monocotyledonous plant). cv, fibro-vascular bundles; gc, pithy material between 

 bundles. B, Diagrams of stem sections (exogens or dicotyledonous plant). a, 

 cross-section of chickweed stem, the inner circle representing the cambium ring, 

 the two radial lines indicating the portion enlarged in b; e, epidermis; h, hair; 

 c, cambium-separating between p, phloem and w, woody portions of bundles; 

 v, spiral vessels in the woody portion; x, pith and y, common parenchyma of 

 bark; c, segment of a sunflower stem; p, parenchyma; b, bast fibres; s, sieve 

 tube; c, cambium; g, vessels, pitted and spiral; h, wood fibres; d, one year, and 

 e, four year old woody stems, illustrating the increase of vascular bundles. (From 

 Needham's "General Biology" after Wettstein, by permission of The Comstock 

 Publishing Co.) 



As no seed can be formed unless the reproductive organs, stamen 

 ( ) and pistils ( ) are present, 



these are called essential organs, and plants having both essential organs 

 in a single flower are called perfect flowers, while those having only one 

 or the other essential organs, are called imperfect. 



If a flower possess, in addition to the essential organs, a calyx 

 ( ) and a corolla ( ), it is called 



a complete flower. 



All of these parts are better understood from a study of Figure 146 

 than from any description which could be given. 



Fig. 126. 

 Three Growth Zones, showing 

 arrangement of the Fundamen- 

 tal Tissue Layers in roots and 

 stems. 1, Dermatogen zone. 2, 

 Periblem zone. 3, Plerom zone. 

 (After C. W. Ballard's "Vege- 

 table Histology," (Courtesy of 

 John Wiley and Sons.) 



PLANT HISTOLOGY 



A correct understanding of plant tissues 

 can, however, come only from a knowledge 

 of how such tissues develop. 



Just as we shall soon see, hydra (because 

 it is composed of tissues only) can regenerate 

 almost any portion of the body, so, too, the 

 early embryonic substance of plants is all 

 quite alike, and can develop into many and 

 varying types of cells. This early undiffer- 

 entiated embryonic plant tissue is known as 

 meristem. It is from this meristem that 



