From the Digest

The Thin Green Line

By Joe Merkelbach (June 2010)

Eric Driskill’s informative presentation on grafting caused me to research a bit into cactus cambium and the special properties that facilitate relatively easy combinations of cacti and succulents.

Cambium layers are areas of meristematic cells that allow the growth of various types of differentiated plant tissues. Meristem cells of plants are analogous to stem cells of animals – they can convert to several functions as they mature.

In perennials, including cacti and succulents, the cambium layer in the stem gives rise to two tissues. Xylem, formed inward from the cambium, conducts water and minerals up from the roots to the stem and leaves (only to the stem for most cacti, as they lack leaves). Phloem, formed outward from the cambium, conducts the sugars made by photosynthesis down the stem to nurture the plant.

Both these tissues are formed of elongated tubular cells. In the case of xylem, it can become lignified or woody, and this rigidity property allows the tall growth of many cacti. The rigid dead xylem tissue both transports water and provides support.

The intricate patterns in the skeletons of cactus wood are due to the rays of interpenetrating water storage tissue. These are readily seen when the soft tissue sloughs away from a dead plant.

The parenchyma cells that do the water storage in cacti are soft, spherical and generally colorless. Since there are so many of them for voluminous water storage, however, they are connected to the primary xylem and phloem by cortical bundles for more rapid transport of water and glucose. These bundles are distributed throughout the cortex in a manner similar to the venation of leaves.

There is another type of cambium layer which differentiates much later in the life of most cacti. It is called cork cambium and forms from the epidermis instead of the usual vascular cambium layer. It forms when the stem surface can no longer photosynthesize but still must protect against moisture loss.

The green color of the cambium is due to the continual formation of new tissue. This propensity for constant growth is also the property that allows successful grafting.