410 THE ANATOMY OF WOODY PLANTS 



axes in the group supplies convincing evidence that the peculiar 

 arrangement of the fibrovascular bundles in the monocotyledons 

 is not a primitive one. The original manner of distribution of 

 the conducting strands of the stem was in all probability that 

 found in the dicotyledons, characterized by a prevailing circular 

 arrangement of the bundles. 



The root in monocotyledons has the usual radial organization, 

 and is distinguished from that of the mass of dicotyledons by the 

 absence of secondary growth. Another feature which is often 

 present in monocotyledonous roots is the origin of lateral roots, 

 not opposite the groups of protoxylem, as is the general situation 

 in the roots of the remaining vascular plants, but in the interval 

 between two protoxylem clusters. This peculiarity has gained 

 for such roots the not very appropriate designation of "double 

 roots." Another feature which has been described in the roots 

 of the group under consideration is found in the abnormal order 

 of development of the elements of the xylem. In vascular plants 

 in general the protoxylem occupying the outside of the xylem 

 star of the root is differentiated first and the successively more 

 central elements in later order. In a number of monocotyledonous 

 roots a remarkable exception to this well-nigh universal seriation 

 of development has been observed, for the more central elements 

 belonging to the metaxylem are differentiated first and the tra- 

 cheids of the protoxylem are the last to manifest the sculptural 

 features of maturity. 



The leaf in the monocotyledons is characterized in general by 

 the closed disposition of the nerves or fibrovascular strands. These 

 usually come together at the tip of the foliar organ and sometimes 

 in this region are in relation to rifts or pores in the epidermis, 

 which allow fluid water to escape in the form of drops of dew. In 

 certain tropical plants with large leaves such, for example, as 

 the Agave the loss of water during the night through the rifts 

 in the tips of the leaves where the foliar bundles converge is so 

 great that a constant dripping is heard, often so pronounced as 

 to disturb slumber. As a consequence of the closed disposition 

 of the fibrovascular bundles in monocotyledonous leaves, only 

 the longitudinal veins are, as a rule, well developed, and the 



