30 T O R R E Y A 



species in some detail from the particular point of view of the problem of cell 

 division. 



Eqiiiscfuni hycinalc, one of the larger of our native species, was chosen 

 for study. This has a rather massive meristematic region as compared with 

 some of the more delicate types. Transverse and longitudinal sections, both 

 median and tangential, were cut through the stem tips which had been col- 

 lected at various times from ]\Iarch until June, the period when meristematic 

 activity and gro^^•th are best studied. Cell divisions occur not onh' at the apex 

 of the meristem near the apical cell, but for some distance back along the 

 axis during the differentiation of various tissues. 



The manner in which division takes place was found to differ markedly 

 depending on the location and character of the cells concerned. Some of the 

 types are as follows : 



The large apical cell cuts off a daughter cell from each of its three inner 

 faces, successively. The new wall is approximately parallel to the old so 

 that the two daughter cells are dissimilar in shape and .usually in size (Fig. 1 ) . 



The lower cell elongates, as seen in longitudinal section, and divides 

 periclinally. The inner of its daughter cells contributes, by rather irregular 

 divisions, to a mass of tissue just below the apical cell. The outer one divides 

 anticlinally. and thus parallel to its long dimension. This portion of the meri- 

 stem thus consists of a surface layer of elongate cells and an inner mass of 

 irregularly-shaped ones (Fig. 1). 



!Most subsequent divisions in the outer layer are anticlinal, thus violating 

 Hofmeister's rule, with the new division wall straight and parallel with the 

 sides of the mother cell. Occasionally, however, usually at the point where a 

 new leaf primordium will develop, the inner edge of the phragmoplast begins 

 a straight course but before it reaches the end of the mother cell it swerves 

 to one side, usually in the basal direction, until it meets the anticlinal wall of 

 the old cell (Fig. 2). The smaller of the two daughter cells thus produced, 

 somewhat \'-shaped in section, will form the apical cell of one of the leaf 

 primordia which begin to appear a little further back. Such a wall as here 

 described is neither across the shorter dimension of the cell nor does it con- 

 form to a least-surface configuration, although in the inner part of its course 

 it becomes curved. 



Shortly below this level there may be seen in longitudinal section a series 

 of cell divisions across the axis. This marks the beginning of one of the 

 diaphragms which is such a conspicuous feature of the stem anatomy of 

 EquisefiiJii. These divisions are always approximately at right angles to the 

 axis regardless of the particular shape of the cells in which they occur and, 

 therefore, occupy positions in these cells which violate many of the "rules." 



