THE ANATOMY OF THE SHOOT 65 



from it. Another fundamental difference exists in the manner of 

 organization of the histogens and the way in which these respective 

 layers undergo cell division. In the stem apex, the surface layer or 

 dermatogen, as well as the one or two layers underlying it, divide 

 anticlinally with the result that they increase in area without 

 adding to the thickness of the axis. On the other hand, the more 

 centrally located cells divide in all planes; but the rate of division 

 is essentially the same in both cases, as demonstrated by Schiiepp 

 (35, 36). As Priestley and Swingle (30) have pointed out, the 

 addition of cells to both the superficial and central regions at the 

 same rate, coupled with the fact that the divisions of the dermato- 

 gen and adjacent layers are only anticlinal, while those of the inner 

 layers occur in all planes, results in the formation of folds by the 

 surface tissues, which become the initials of the new leaves. The 

 further development of leaf primordia is discussed in a later 

 section. 



Deferring, for convenience in description, the discussion of the 

 formation of the lateral members, the ontogeny of the stem axis 

 may best be understood by considering the progressive development 

 of the stem as represented at successive levels from its apex to a 

 point where secondary thickening is initiated. At the apex, the 

 meristematic growing point consists of thin-walled parenchyma- 

 tous cells which are relatively small with a dense cytoplasm and 

 prominent nuclei. These cells divide actively, and there is no 

 evident differentiation among them except for changes in the shape 

 of the cells of the outer layer (dermatogen) which, because of their 

 position and internal pressure, may become tangentially elongated. 

 It is in this region that the superficial folds occur which are the 

 forerunners of leaf primordia. 



Somewhat below this level, the differentiation of the provascular 

 strands occurs. In herbaceous dicotyledons, these form a pro- 

 cambial cylinder in which the strands consist of groups of small 

 cells that continue to divide while the adjacent ones increase in 

 size and become more vacuolate. As progressive development oc- 

 curs, the vascular cylinder becomes more sharply delimited owing 

 to the increased growth and vacuolation of the parenchymatous 

 tissue of the pith, cortex, and rays; and the appearance of larger 

 intercellular spaces in these regions. The cells of the procambial 

 strand continue to divide, and each strand may become somewhat 

 extended tangentially as a result of such meristematic activity. 



