THE PLANT BODY 



33 



consists of the remaining central tissues in which cell divisions are in 

 various planes and little or no peripheral layering is present. The mantle 

 may add to the core. 



In somewhat modified definitions, the corpus is the central core of 

 the apex, consisting of larger cells, varying in form, without definite 

 peripheral layering, and with cell divisions in many planes; the tunica 

 consists of one to several outer uniseriate layers of smaller cells, uni- 

 form in size, with divisions wholly or largely anticlinal (Fig. 12). In 

 the corpus, cell divisions are infrequent, and the protoplasts stain 

 lightly; in the tunica, divisions are more frequent and the cytoplasm 

 stains deeply. Tunica and corpus, though not always sharply delimited, 

 are independent, self-perpetuating meristems. The chief variations are 

 in number of layers in the tunica — at first 

 considered constant for a species — and 

 in the clearness of separation of tunica 

 and corpus. The major difficulty in de- 

 limiting tunica and corpus has been the 

 interpretation of the outer layer of the 

 corpus, a layer, with frequent anticlinal 

 divisions, which may be more or less dis- 

 tinct as a uniseriate sheath and appears 

 transitional between corpus and tunica. 



Below the tunica-corpus zone in the 

 apex, there may be distinguishable a 

 region of transition between the highly 

 meristematic distal cells and the maturing 

 cells, in which enlarging cells of like type 



stand in long rows or files. This zone has been called a third meristem, 

 a. rib meristem (Fig. 12). 



A rather broad survey of shoot-apex structure in various tribes of 

 the Rosaceae seems to show that clear tunica-coipus zonation is not a 

 characteristic feature of tliis family. Absence of definite zonation here 

 has been considered a surprising departure from the apex structure 

 described for taxa throughout angiosperms and as perhaps the result of 

 differences in interpretation. But dehmitation of tunica and corpus may 

 be weak or obscure. In tlie monocotyledons, rather few shoot apices 

 have been studied, perhaps because of greater complexity of structure 

 in these plants. 



The tunica is probably most commonly two-layered. A one-layered 

 tunica is common in monocotyledons and occasional in dicotyledons. A 

 few major taxa have mostly several-layered tunicas — Compositae, 

 Caprifoliaceae, Rosaceae, Guttiferae. Variability in number of layers is 

 more frequent in dicotyledons than in monocotyledons. According to 



Fig, 12. Zonation diagram of a 

 hypothetical shoot apex showing 

 direction of cell divisions. 1, tu- 

 nica; 2, corpus; 3, rib meristem; 

 4, flank meristem. (After Stant.) 



