156 



LECTURES 



Fig. ii. 



Fig. 13. 



on its edges, as in the cycas family. Figs. 12 and 13 represent cross 



sections of the capiUary leaves of naked 

 seeded plants. The Angiosperms, on the 

 contrary, bear their seeds enfolded within 

 the capillary leaf or seed vessel, (figs. 14 and 

 15,) as in all the ordinary flowering plants. 

 The Angiosperms are again subdivided into 

 3fonocolyledons, (one cotyledon or seed leaf 

 in the embryo,) fig. 16, and Dicotyledons, 

 (t wo seed leaves in the embryo,) fig. 17. 



Fig. 16. 



Fig. 17. 



Cryptogams, 



( Dicotyledons, } k - 



Pheenogams, ] Monocotyledons, \ Angiosperms. 

 ( Conifers & Cycada^^ Gymnosperms. 



Vascular Cryptogams. 

 Cellular Cryptogams. 



Now, the most important means of determining the families of coal 

 plants are the internal structure of the stem and the venation of the 

 leaves. Generally, indeed, these are the only means at oi'ir command. 

 Let ns inquire, then, how the great divisions of the vegetable king- 

 dom are characterized in these respects. 



Among Phcenogams there are two very distinct types or plans of 

 internal structure of the stem, viz : the Exogenous, or outside-growing, 

 and the Endogenous, or inside-growing ; the one represented by the 

 hard-wood trees and shrubs, the other by the palms, canes, grasses, 

 &c. On cross section of an exogen (fig. 18) we find three distinct 

 zones of tissue. In the centre a zone of cellular tissue, the pith ; 

 exterior to this a zone of wood, and around this again a zone of cellular 

 tissue, the bark. The zone of wood is, moreover, subdivided into con- 

 centric rings, which represent the annual layers of growth, and sepa- 

 rated into wedges by radiating lines of cellular tissue (silver grain) 

 connecting the cellular tissue of the pith with the cellular tissue of 



the bark. In the Endogens, 

 on the contrary, we have the 

 woody tissue in the form of 

 thread-like bundles, irregu- 

 larly interspersed amongst 

 the cellular. The dry stalk 

 of an Indian corn is a fa- 

 miliar illustration of this 

 structure. If such a stalk is 

 broken across and the two 

 parts careiuUy separated , the 

 thread-like bundles of woody and vascular tissue are observed to draw 



Fig. 18 



