1 9 i 2] JONES— DIANTH ERA 5 



For ease of description, we will divide the growth of the seedling 

 into stages corresponding to the number of pairs of leaves present. 



STAGE I 



The first seedling stage (fig. 15) shows the elongating hypocotyl 

 well out of the testa, pushing upward the cotyledons which are 

 frequently still inclosed within the testa. All the endosperm 

 found in the seed has been used up. The primordia of the first 

 pair of leaves are beginning to develop, but no sign of differentiation 

 of the foliar traces has yet appeared. From each cotyledon one 

 double bundle enters the hypocotyl (fig. 16). These bundles 

 approach each other, and very soon come together to form the 

 central cylinder. 



In the middle of the hypocotyl, a cross-section shows an epider- 

 mis of cells slightly elongated radially, the inner and side walls 

 thin, the outer walls slightly cutinized; the cortex, about 10-12 

 cells in thickness, the outer two or three layers of irregularly 

 arranged cells, which are beginning to show a slight thickening and 

 later forming a typical collenchyma. The remaining inner layers 

 of the cortex are made up of rounded thin-walled cells, very regu- 

 larly arranged in radial rows, their walls being in contact except at 

 the angles, where there are formed small schizogenous air cavities, 

 which latter extend vertically for a considerable distance. The 

 innermost layer of the cortex is not modified in any way, being like 

 the other cells of the inner part of the cortex in size, shape, and 



content of cells. 



The central cylinder, or stele, is very sharply marked off from 

 the surrounding cortex, being made up of much smaller cells except 

 at the very center. The pericycle is of one layer except opposite 

 the xylem poles, where it is of two layers. The xylem is arranged 

 in two opposite groups, the protoxylem being exarch. The four 

 phloem groups are placed one on each side of the xylem poles. The 

 rest of the central cylinder is of thin-walled parenchyma, small 

 near the periphery, but becoming much larger at the center. There 

 are no air spaces in the central cylinder. 



Transition. — In the petiole of each of the cotyledons is a double 

 bundle, the protoxylem of which occupies an endarch position. 

 As the two bundles of the cotyledons enter the hypocotyl, there is 



