THE ANATOMY OF THE SHOOT 79 



general uniformity in the details of development, and there is a lack 

 of agreement as to the terminology applied in ontogenetic descrip- 

 tions. For this reason, it seems desirable to summarize briefly the 

 situation with respect to nomenclature before discussing the details 

 of leaf development. 



Hanstein (x5) applied the same terminology to the meristematic 

 tissues of the stem tip as to the root tip, with the exception of 

 the calyptrogen ; and described the former as being organized into a 

 dermatogen, periblem, and plerome. According to his account of 

 leaf development, lateral members originate below the tip of the 

 growing point where definite groups of meristem are formed involv- 

 ing elements of both the dermatogen and periblem. Subsequent 

 divisions of the dermatogen are anticlinal only so that it forms a 

 continuous, single layer covering the leaf, which later develops as 

 the epidermis. The underlying cells of the periblem, by divisions 

 in all planes, develop the tissues of the mesophyll, including the 

 vascular bundles. The plerome of the stem axis does not enter into 

 the formation of the lateral structures, either leaf or branch. 



More recent studies indicate that there are exceptions to this 

 type of ontogeny; and that the number of layers of meristematic 

 cells which may be involved in leaf development is variable, rang- 

 ing from one to several . Rosier (3 1) found that the leaf of Triticum 

 develops entirely from the dermatogen, and Priestley and his 

 associates (19) observed periclinal divisions in the dermatogen as 

 well as underlying layers in some of their studies. On the other 

 hand, subepidermal layers of the apical meristem of the shoot may 

 be responsible for the development of the leaf tissues with the 

 exception of the epidermis, and several different types of sub- 

 epidermal activity have been reported. In some instances, the 

 third layer of cells appears to be responsible for the development 

 of the midrib or a portion of it, while the remainder of the meso- 

 phyll arises from the subepidermal layer. In Linum, the leaf 

 primordium involves at least the three outermost layers, according 

 to Crooks (9); while in tobacco, Avery (5) states that 



"the leaf arises as a lateral projection of the embryonic stem tip. 

 Its initial impetus for development comes from a few localized dividing 

 and enlarging cells in the outer layers of the promeristem, in the 

 portion which becomes the periblem, and still later cortex." 



Because of the difficulty in following the ontogeny of the leaf 

 on the basis of the histogens as used by Hanstein (15), Schmidt (33) 



