24 MERISTEMS 



initiation of leal: and bud priniordia (Foster, 1936). While the 

 outermost tunic layer very eonnnonly behaves as a "dermatogen" 

 and produces exclusively the epidermal system of leaf and stem, 

 the apices of Triticuin and AL'cna furnish interesting- exceptions. 

 In these grasses, the foliage leaf originates largely if not exclu- 

 sively from the single tunic layer which exhibits both perielinal 

 and anticlinal divisions at the early phases of foliar develop- 

 ment. Doubtless similar conditions will be discovered in other 

 angiosperms. 



A proper study of the form and structure of the shoot apex 

 in seed plants and of the origin of primary stem tissues and 

 leaves is only possible if both longitudiiidl and iniusversc serial 

 sections are available. .Since the choice of bud material will 

 depend upon many factors, no detailed description of a specific 

 shoot apex will be made in this book. Instead, suggestions as 

 to the advantages and special features of several available types 

 of apices will be given. With the information presented in llie 

 earlier portions of this exercise and in the literature cited, the 

 student should have no difficulty in interpreting the general 

 organization of any angiospermous shoot apex. The mound- or 

 dome-shaped form of apex, with several tunic layers, is well illus- 

 trated in such genera as Carya, Morns, Rhododcndroru Acacia, 

 Syringa, Rosa, Sanibucus, and Hch'anthus. Aside from minor 

 variations, the origin of leaves and the differentiation of provas- 

 cular strands ("pro-cambium") and "i-ib meristem" are simi- 

 larly shown in all of these genera. "/>'//> iiKrishiii/' a coiu-ept 

 developed by Schiiepi) (1926), is a type of primary mei-istem 

 which in a hmgi-seetional view of a shoot apex appears as a tissue 

 composed of vertical filamentous gi-oujjs of vacuolating-dividiug 

 cells. This mci'istem typically differentiates into the paiHMU'hyiu;i 

 tissue of cortex and pith. The slender cone-shaped apices of 

 Elodea or Hippuris are instructive, providing median longi-sec- 

 tions are examined. Apices of these genei-a are jiarticnlarly use- 

 ful in demonstrating the mode of origin of the small leaf ]n-i- 

 mordia fi-om the tunica zone, as well as showing the early demar- 

 cation between cortex and the pith-less stele. Preparations of 

 the shoot apices of monocotyledons should also be studied. The 

 apex of Tradcscaiilia is of intei'cst since the (leinarcation betAveen 



