The following points highlight the two main forms of combial cells in plants. The forms are: 1. Storied or Stratified Cambium 2. Non-Storied or Non-Stratified Cambium.
Form # 1. Storied or Stratified Cambium (Fig. 23.1B):
In TLS the initials are arranged in horizontal regular file having a stratified structure. The initials are usually short and all of about the same length. The ends of the initials occur approximately at the same level. The length of initials ranges from 140 µ to 520 µ. Ex. Wisteria, Tamarix, Robinia etc.
Form # 2. Non-Storied or Non-Stratified Cambium (Fig. 23.1A):
In longitudinal tangential view the tapered ends of fusiform initials overlap each other in a random arrangement. Ex. Rhus. Non-storied cambium is more common and longer than the storied type.
The length of the initials ranges between 320 µ to 2300 µ and it may be as long as 6200 µ, e.g. vesselless dicotyledons. Initials of 1000 µ to 8700 µ are reported from gymnosperm. It is regarded that these initials, during phylogenetic evolution gave rise to storied initials by reduction.
Ray initial:
The term ray initial refers to a group of initial cells. In contrast fusiform initial refers to a single cell. A single cell of ray initial is referred to as ray cell initial.
The ray initials compose the radial system of cambium. A ray cell initial is roughly cuboidal or isodiametric in shape and much smaller than fusiform initial. At the time of cell division the radial dimension becomes greater than the longitudinal dimension. The radial dimension also becomes greater than the width of fusiform initials.
Ray cell initial together with its derivative rays may be uniseriate, biseriate or multiseriate depending upon one, two or more longitudinal rows of cells composing them. The height, shape and the number of ray cell initial vary. Ray initial is best studied in TLS and RLS. In TLS multiseriate ray appears to be fusiform.