THE ANGIOSPERMS 377 



significance of chalazogamy from the standpoint of fibrovascular 

 anatomy. It seems, however, a feature too likely to be modified 

 somewhat rapidly by conditions to rank as a criterion of the first 

 order for the distinguishing of the most primitive dicotyledons. 



The monocotyledons are distinguished, as their name indicates, 

 by a seed containing an embryo with a single cotyledon or seed 

 leaf. This feature is very constant, but there are indications of 

 the presence of a second cotyledon in certain of the grasses, such 

 as Zizania, Avena, etc. The fibrovascular strands of this group 

 are ordinarily closed; that is, they do not possess the capacity to 

 increase in thickness through the activity of a cambial layer. 

 The arrangement of the strands in the monocotyledonous angio- 

 sperms is also distinctive, since the bundles, instead of being 

 disposed in a circular fashion as in the mass of herbaceous dicotyle- 

 dons, are scattered throughout the transverse section of the cylinder 

 and sometimes even occur in the cortex. This peculiar disposition 

 is, beyond any reasonable doubt, the result of the entrance of 

 numerous leaf traces into the axis at each node, a consequence of 

 the high assimilative efficiency of the foliar organs. The veins 

 of monocotyledonous leaves are distinguished primitively by a 

 closed arrangement; that is, starting out at the base of the leaf 

 as a closed system, they reunite at the apex of the leaf. This 

 disposition of the skeletal tissues of the leaf makes it immune 

 from tearing action. The lateral veins, in consequence of this 

 situation, are largely abortive. In many palms, aroids, and 

 Scitamineae the venation of the leaf becomes open as a result of 

 changes in the apex. In such cases the venation of the early- 

 formed leaves of seedlings is closed, showing that this condition 



t 



is the primitive one for the group. The root in monocotyledons 

 is distinguished, as is the stem, by the absence of secondary growth. 

 The bundles are usually distributed in a circular and radial fashion, 

 but in certain palms and orchids they may be scattered throughout 

 the transverse section of the organ as they are in the stem. The 

 floral parts occur in multiples of three, and the floral envelopes, 

 as in the dicotyledons, show less numerical variability than do 

 the essential whorls consisting of stamens and carpels. Pollination 

 is effected usually through the agency of insects, but may be 



