Meristems 87 



interfascicular cambium may become active and produce typical xylem 

 and phloem. Its development offers some interesting morphogenetic 



problems. 



The distribution of cambial activity over the axis in woody plants also 

 deserves further investigation. The amount of wood produced by the two 

 branches at a given fork of a stem, for example, bears a rather close ratio 

 to the amount in the main axis below them, but this ratio will depend on 

 the relative size of the two branches, the angle between them, and the 

 orientation of the main axis itself ( p. 108 ) . 



Another relatively unexplored field but one which may become of much 

 interest for morphogenesis is that of anomalous secondary growth. In 

 certain families the normal situation of a continuous cambial sheath is 

 altered (Mullenders, 1947). An additional cambium may arise outside the 

 phloem and start another vascular cylinder or series of bundles. There 

 may be more than one of these. Such anomalous bundles may also appear 

 in the pith. In other cases the surface of the cambium, instead of being 

 circular in cross section, may be irregular so that radial lobes of secondary 

 tissue are formed. In more extreme cases the cambium may become quite 

 atypical and patches of secondary xylem and phloem may be intermingled 

 in the vascular cylinder. Anomalous growth is often found in stems such 

 as those of lianas or rhizomes which have other functions than support 

 or conduction. It is frequently present in fleshy roots. One sometimes has 

 difficulty in drawing a line between anomalous growth of this sort, 

 which is really normal for a particular plant, and truly abnormal, or tera- 

 tological, structures (Chap. 11). 



The vascular cambium has been little explored from a morphogenetic 

 viewpoint. Although it is much more difficult to study directly than are 

 the apical meristems, its products, especially wood, are so firm and meas- 

 urable that they offer attractive material for a quantitative study of many 

 problems in growth relationships. 



The Cork Cambium. The vascular cambium and the root and shoot 

 apices are not the only localized embryonic regions in the plant. Increas- 

 ing diameter of the axis necessarily results in the rupture of its outer 

 layers, notably the cortex and the outer phloem. Infection and water loss 

 would take place through these breaks in the tissue were it not for the 

 formation of layers of suberized cells, the cork, or phellem. This is sec- 

 ondary tissue formed by a cork cambium, or phellogen. It has its origin in 

 a row of cells, tangentially adjacent to each other, which divide peri- 

 clinally and link up into a meristematic layer that produces a series of 

 daughter cells on its outer side. There may be from one or two to many 

 of these and their walls become suberized and impervious to water. On 

 the inside are formed one or a few layers of daughter cells, the phello- 

 derm, presumably vestigial in character. 



