82 



STRUCTURAL BOTANY 



and root is also complete. The change from the 

 epidermis of the stem to the piliferous layer of the 

 root is, however, a sudden one. 



e. Apical Development 



We will now describe rather more fully how the 

 development of the Wallflower plant takes place. 

 We already know that the growing points, i.e. the 

 regions where the formation of new 

 organs and tissues goes on, lie at 

 the extreme ends of the branches 

 of both shoot and root. In every 

 branch the base is the oldest, the 

 apex the youngest part. It is at 

 the apex, then, that we shall find 

 development in progress. 



(a) The growing point of the 



FIG. 32. Growingpoint 

 of a stem in longi- 

 tudinal section. 

 I, young leaves; 

 ep, young epidermis, 

 or dermatogen ; i, 

 periblem, or layer 

 from which the cortex 

 is developed ; pi, ple- 

 rome, which develops 

 into the central cylin- 

 der. (After Douliot. ) 

 Magnified. 



shoot. If we examine a longi- 

 tudinal section passing exactly 

 through the middle of a growing 

 point, we find that the end of the 

 stem has a nearly flat or slightly 

 domed surface. If the section has 

 passed through one or more of the 

 very youngest leaves, we shall see 

 that they arise as slight projections on the sides of 

 the growing apex. Lower down, we come to rather 

 more advanced leaves, which at first simply show an 

 increase in size ; while farther down still, their tissues 

 begin to be differentiated. 



