CHAPTER XVII 



COMPARATIVE ANATOMY OF ANGIOSPERMS* 



THE question of the relationship of the two great divisions 

 of the Angiosperms has for many years been a matter of dis- 

 pute. Anatomically the differences between the Dicotyledons 

 and Monocotyledons are sufficiently well marked, but it has 

 not been easy to decide from ordinary anatomical data which 

 should be regarded as having the more primitive and antece- 

 dent organization. There can be little doubt that the two groups 

 are closely related, for in addition to the striking general re- 

 semblance of their sporophytic tissues there is almost an identi- 

 cal organization of the male and female gametophytes. The 

 Monocotyledons have by some been regarded as primitive on 

 account of the absence of a cambium in their ordinarily closed 

 bundles. This view has been strengthened by statements as to 

 their appearing earlier in the geological strata than the Dicot- 

 yledons. It is now known beyond doubt, however, that many 

 of the earlier cryptogamous groups had well-marked secondary 

 growth, so that the absence of cambial activity is by no means 

 necessarily a primitive feature. Further, a more careful study 

 of plant fossils has made it clear that many of the remains for- 

 merly considered to be Monocotyledons are in reality Pterido- 

 phytes or Gymnosperms. Discussion of these interesting prob- 

 lems will be more profitable after the salient features of the 

 anatomy and development of the Angiosperms have been de- 

 scribed. 



DICOTYLEDONS 



It has been shown by Jeffrey 13 that the primitive condition 

 of the central cylinder in the Angiosperms is siphonostelic. 

 The tubular central cylinder of the seedling of Ranunculus, 



* Contributed by Professor Edward C. Jeffrey, of Harvard University. 

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