ZEA MAYS 119 



node, grow upward until they emerge through the pericarp and 

 they then incline downward. Later a whorl of adventitious roots, 

 four or more in number, originates just above the level of the second 

 node. (Fig. 45.) 



The Seedling Axis. — The seedling axis has been variously in- 

 terpreted by anatomists working with the grasses. Celakovsky 

 (4) considered the scutellum as homologous with the blade of a 

 leaf and the coleoptile as comparable to the closed ligule of the 

 foliage leaf of the grasses. He regarded the portion of the axis 

 between the scutellum and the coleoptile as an elongated node and 

 called it the mesocotyl. Worsdell (2.6) agreed in part with Cela- 

 kovsky and considered the mesocotyl an elongated primary node. 

 He interpreted the scutellum as the lamina of the cotyledon, and 

 the coleoptile as being a part of the cotyledon comparable to the 

 ligule of a foliage leaf. He also stated that "the position of the 

 cotyledon in all monocotyledons as shown by the facts of develop- 

 ment, there being no epicotyledonary axis present on its first forma- 

 tion, is always terminal, and is the natural continuation and 

 termination of the hypocotyl." Sargant and Arber (16) regarded 

 the scutellum as the sucking apex of the cotyledon, the coleoptile 

 as the cotyledonary sheath, and the mesocotyl as a unique structure 

 representing a fusion of the cotyledonary stalk with the hypocotyl. 

 Weatherwax (xi) pointed out that the embryogeny of maize in- 

 dicates "that the coleoptile is the homologue of a foliage leaf and 

 that the cotyledon is a lateral organ." Avery (i) stated that the 

 scutellum is the cotyledon, "the coleoptile is homologous with a 

 foliage leaf and is the second leaf of the plant. . . . The elongated 

 structure between the cotyledon and the coleoptile is the first inter- 

 node of the axis . ' ' This interpretation appears to be in accord with 

 the anatomy of the corn seedling; and, under it, the use of the 

 term mesocotyl is without point. 



The Primary Root. — The mature primary root has an exarch, 

 radial siphonostele with a central pith composed of large paren- 

 chymatous cells. In the embryonic axis, three distinct histogens 

 are differentiated in the meristematic growing point of the primary 

 root. (Fig. 46.) The plerome, or innermost of these histogens, 

 produces the tissues of the stele and included pith. The periblem- 

 dermatogen consists of a single layer of meristematic cells overlying 

 the plerome. Periclinal divisions of the lateral cells of this his- 

 togen produce two daughter cells, the outer daughter cell becoming 



