i9i THE STRUCTURE OF ECONOMIC PLANTS 



thickened. They are gold or brown and appear reticulate and 

 sharply polygonal in surface view. (Fig. 144, D, £.) 



The fourth or pigmented zone is one or more cell layers in 

 thickness and contains the coloring matter of the coat. It con- 

 sists of much-compressed cells with extremely thin walls, which 

 in some varieties are small, but in others, including the Scarlet 

 Globe, are large and polygonal with an abundant dark-brown 

 pigment. 



The remaining layers have been interpreted by some investigators 

 as a part of the inner integument, but Winton and Kondo have 

 demonstrated that they are portions of the endosperm. The aleu- 

 rone cells of this zone are somewhat similar to those found in the 

 outer portion of the endosperm of cereals. This layer is usually one 

 cell in thickness except in the region of the micropyle, where there 

 may be two or more layers, and adjacent to the hilum where it may 

 be entirely absent. The remainder of the endosperm consists of a 

 hyaline zone made up of several layers of tangentially stretched 

 and much crushed parenchymatous tissue. 



Development of the Seedling. — The germination of the seed 

 is rapid. Seedlings have been observed with from six to eight 

 laterals on the upper inch of the tap root within four days of plant- 

 ing and before the cotyledons are completely unfolded. At the 

 end of nine days, when the first foliage leaves appear, secondary 

 laterals have been formed. The primary root pushes through the 

 seed coat near the micropyle and elongates rapidly, becoming 

 slightly arched at its upper limits. In the early stages of develop- 

 ment, elongation of the hypocotyl and root proceeds much more 

 quickly than in the epicotyl. The growth and straightening of 

 the hypocotyl lifts the conduplicate cotyledons above ground, and 

 they soon expand to form the first photosynthetic organs of the 

 seedling. (Fig. 145.) 



Ontogeny of the Root. — The primary root has a diarch pro- 

 tostele. The cells of the two protoxylem points are differentiated 

 centripetally, and with the larger metaxylem vessels toward and at 

 the center, form the complete primary xylem strand. The two 

 groups of primary phloem alternate with the points of protoxylem 

 and are separated from the xylem by a zone of fundamental paren- 

 chyma. The pericycle consists of a single layer of cells lying imme- 

 diately within the endodermis in which Casparian thickenings are 

 evident by the time the primary xylem is completely matured. 



