48 



MORPHOLOGY OF THE ANGIOSPERMS 



in the secondary xylem, then, progressively, in metaxylem and pro- 

 toxylem. Advance in speciaUzation in wood has "worked backward" 

 from later to earlier and earlier formed tissue. In the monocotyledons, 

 vessels clearly arose first in the later-formed metaxylem, then in earlier- 

 formed metaxylem and protoxylem. Vessels did not appear simul- 

 taneously throughout the plant body. In the monocotyledons at least, 

 they arose first in the roots and then developed successively in higher 



Fig. 24. Transverse (A) and tangential (B) sections of part of a stem of Amhorella 

 trichopoda showing vesselless xylem with uniseriate, biseriate, and triseriate rays. 

 (From Bailey and Swamij.) 



and higher parts of the plant. All steps in this phylogenetic "spread" of 

 the vessel through the plant body are found. In the primitive Alismata- 

 ceae, they are present only in the roots; in the highly advanced Grami- 

 neae, they are present throughout the plant. The origin of the vessel in 

 the monocotyledons is perhaps comparatively recent. 



Vessels in angiosperms arose without question from scalariformly 



