X98 



THE STRUCTURE OF ECONOMIC PLANTS 



mesophyll consists of three to five rows of palisade cells and the 

 spongy parenchyma occupies a zone as thick as the palisade region, 

 being made up of cells that are compactly arranged, so that the 

 intercellular spaces are very small. Stomata are more abundant on 

 the lower than on the upper surface. (Fig. 154, F.) 



Secondary Thickening of Primary Root and Hypocotyl. — 

 Secondary thickening begins early in ontogeny; and approximately 

 at the time that the first foliage leaf appears, the hypocotyl and 



upper portion of the root 

 begin to enlarge. This in- 

 crease results from the growth 

 of the stelar portion of the 

 axis, since the cortical and 

 epidermal cells do not divide 

 or enlarge to any great extent 

 after the primary tissues of 

 the stele are completely differ- 

 entiated. As a result, there 

 is a rupturing of the epidermis 

 and cortex, forming two lon- 

 gitudinal splits that arise in 

 the transitional region of the 



Fig. 148. A-D, diagrams showing stages in hypOCOtyl below the COty- 

 vascular transition : A, at root level ; B, lower , , , , , 



hypocotyl; c, middle hypocotyl; D, upper ledonary plate and extend 



hypocotyl. Stippled portions represent phloem; down tOW^atd the roOt. The 

 cross-hatched areas protoxylem; lined areas, ^ ,^ ^ ^^^ j. • 



metaxylem. (After Grassley.) y 



tical plane which is median 

 between the two cotyledons; and as enlargement continues, these 

 are extended nearly to the cotyledonary node. Two flap-like seg- 

 ments of partially split off cortical tissue may remain in the plane 

 of the cotyledons until the fleshy axis has reached its maximum 

 size. (Fig. 150.) 



Except for the green upper portion, there is little axial elongation 

 of the hypocotyl after the maturation of the primary tissues unless 

 there are unfavorable environmental conditions, such as dryness 

 or light deficiency, which delay the initiation of secondary thicken- 

 ing. In such cases, the entire hypocotyl elongates considerably 

 and there may be little or no secondary thickening. Temperature 

 may also have a pronounced eff^ect upon the development of the 

 hypocotyl, and Plitt (13) found that under short day conditions 



