THE ANATOMY OF THE SHOOT 67 



which becomes the sieve tube, the other the companion cell. In 

 general, a series of adjacent cambial cells divides simultaneously, 

 and the daughter cells on one face differentiate into xylem or 

 phloem respectively. The remaining daughter cells continue as 

 cambial cells; and in this manner the cambium is perpetuated. 



A further effect of the simultaneous division of adjacent cambial 

 cells is that the elements formed from them lie in radial rows which 

 may be very regular in their alignment. Variation in the subse- 

 quent development of the vascular elements, together with division 

 of some of the derived cells, may disturb the radial symmetry of 

 the secondary tissues, this being especially true in herbaceous stems. 

 In addition to the tangential division of the cambial cells, divisions 

 occur in other planes so that the cambium maintains itself as a 

 continuous layer. Thus radial divisions take place which com- 

 pensate for the increase in the circumference in the axis as secondary 

 thickening proceeds. As Bailey (8) has pointed out, there are two 

 general types of cambial activity which account for the increase in 

 girth of the cambium. In pteridophytes, gymnosperms, and some 

 of the dicotyledons that are less specialized structurally, 



"the anticlinal divisions are more or less transverse and the products 

 of these divisions elongate and crowd by one another, producing 

 thereby an increase in the girth of the cambium and a non-stratified 

 arrangement of its cells. In certain of the more highly differentiated 

 dicotyledons, on the other hand, the anticlinal divisions are radio- 

 longitudinal and the products of these divisions expand laterally, 

 thereby increasing the circumference of the cambium, but they do 

 not elongate to any considerable extent, and thus become symmetrically 

 grouped in parallel, horizontal series." 



The cambium initials are not all of the same size in a given plant, 

 and Bailey (6, 7) has noted that the elongated fusiform initials 

 may vary greatly in their nucleo-cytoplasmic ratio, while this is 

 relatively constant in the smaller ray initials. The latter are much 

 shorter than the fusiform initials whose derivatives mature as 

 elongated vascular elements. With respect to the rhythm or 

 sequence of formation of secondary xylem and phloem elements by 

 the cambium, no specific statement can be made, except that the 

 process is not necessarily a reciprocal one in which there is an 

 alternate development of such elements. In general, the amount of 

 secondary xylem produced exceeds the secondary phloem, which 

 would mean that several successive layers of xylem mother cells 



