INTRODUCTION 



35 



Grass (Fig. 20) may be taken as a familiar instance, 

 the axis being supported and enclosed by the rolled 

 leaf sheaths. These plants form the great division of 

 Monocotyledons or Endogens. 



In a second series of plants the necessary stiffness 

 is given (Fig. 21) by the thickening and hardening of 

 the midrib. In this case the axis is not enclosed, as in 

 the former, by the roUed-up frond, and the stem, not 



Fig. 19. — Frond, strength- Fig. 20. — Portion of a grass stem ; successive internodes 

 ened by rolling. are surrounded by the leaf-sheaths (s) ; I, ligule. 



being surrounded by the leaves, is able to enlarge and 

 expand by successive additions. Fig. 22 represents 

 diagrammatically part of the stem of such a plant as 

 a Thistle, and it is obvious that the midrib of the leaf 

 is an included part of the stem. Hence arises the 

 characteristic secondary growth of the exogenous stem. 



In the first series it is obvious that the leaves are 

 necessarily successive. Consequently the plant is mono- 

 cotyledonous. On the contrary, in the second, two leaves 

 may, and in seedlings generally do, grow simultaneously, 



