THE SEEDLING AND YOUNG PLANT 41 



petal order from these points. In the young stem 1 1n- 

 exact converse occurs the first spiral vessels arise near 

 the centre of the stem, and development proceeds cen- 

 trifagally from the first. We may begin our study of 

 the shoot by tracing the course of the vascular bundles, 

 which, it must be remembered, are the channels of com- 

 munication between the water-supply at the roots below 

 and the leaves and young parts of the shoot above. 



If we cut a transverse section of the terminal bud of 

 the oak, as close to the tip as possible, we shall obtain a 

 preparation of the young axis consisting entirely of 

 embryonic tissue, all the cells of which are practically 

 alike small, polygonal, thin-walled cells, with large 

 nuclei and much protoplasm, but without sap-vacuoles ; 

 these cells are in a state of active division, those in the 

 interior dividing successively in all planes. Those which 

 form the peripheral layer, however, are already distin- 

 guished by only dividing in the two planes at right 

 angles to the periphery, and they constitute the primi- 

 tive epidermis. There is no structure corresponding 

 to a root-cap. 



Transverse sections a little lower down show dif- 

 ferences of the following nature. In the first place the 

 outline of the section tends to be somewhat pentagonal, 

 the points of origin of the very young leaves being at 

 the angles of the pentagon in accordance with their 

 phyllotaxis i.e. the order in which the leaves are 

 arranged 011 the stem. This is of such a nature that 

 each leaf stands some distance above and to one side of 



