What Are the Propagation Methods for Cacti? A Detailed Guide to Common Methods
When it comes to propagation, we must clearly distinguish between asexual propagation by cuttings or grafting and sexual propagation by seed. Through asexual propagation, plants with exactly the same genetic makeup as the parent can be obtained; the offspring is in fact a part separated from the parent. When a cutting is taken, because many cells are cut open and cell sap pours out, the fresh cut surface of the cutting is moist. To prevent the injured plant from transpiring too much water, the wound quickly produces callus tissue to seal itself. Once the cut surface and the nearby cell layers dry, the wound stops evaporating water. After a few days, the still-living cells below the cut surface form a cork layer, and the cut surface then heals and seals completely. When necessary, any living cell can form a cork layer, but only certain specific cells of the cut surface can produce roots, namely a small group of cells in the vascular bundles, the so-called cambium. The cambium lies between the xylem and the sieve tubes, both of which are produced by division of cambium cells. Because the vascular bundles in the middle of the plant are arranged in a ring, the roots produced by a cutting are usually also arranged in a ring. Grafting is completely different from cuttings. If the cut surface of the graft union dries out completely, the scion cannot survive; the scion should be put in place as soon as possible while the cut-surface cells of both the rootstock and the scion are still alive, so that the rootstock and scion fit tightly together. Grafting is actually not very difficult, and whether grafting succeeds also depends on the vitality of the cambium cells. Grafting succeeds only when the cambium of the rootstock and the scion (at least part of it) can heal together.
Pictured: cactus seedlings
If seed is used for sexual propagation, the genetic traits of the two parent plants are recombined. The plant that provides the pollen is called the father parent, while the plant that produces the seeds is called the mother parent. The offspring will not be exactly the same as either of its parents. Because the parents differ in homozygosity, the offspring are also diverse.
We already have some understanding of the structure of cactus flowers and the way they are pollinated. But the true fertilization process of the flower can only be seen clearly under a microscope. Pollen consists of many extremely tiny pollen grains. The outside of a pollen grain has a hard shell, whose main component is sporopollenin, which is highly resistant to decay. This shell is formed by cells separating outward, just as the cuticle is formed by the separation of epidermal cells. Under the microscope, the shells of many pollen grains show beautiful patterns. Pollen grains often live only a few days, but in a cool, dry environment they can survive somewhat longer, which we will discuss in more detail in cactus cultivation. When a pollen grain lands on a suitable stigma lobe, its shell cracks at a thin spot (the germination pore), the cellulose inner wall beneath begins to swell, and a pollen tube gradually grows out of the pollen grain. At this point, the pollen grain needs only the sticky fluid secreted by the stigma lobe in order to germinate.
At the tip of the pollen tube there is a so-called "growth nucleus" (vegetative nucleus), which controls the growth of the pollen tube. As the pollen tube grows, the "growth nucleus" (vegetative nucleus) also keeps moving forward, giving the pollen tube its tip-growth characteristic. The pollen tube travels through the style to the ovule in the ovary. The ovule has two integuments, and the integument bears a small opening (the micropyle) , through which the pollen tube enters the embryo sac. The reproductive cell at the tip of the pollen tube divides into two sperm, one of which fuses with the egg cell right next to the micropyle. This process is called fertilization. At this point, the genetic traits of the father parent and mother parent are recombined. The other sperm fuses with the secondary nucleus. The phenomenon in which two sperm fuse respectively with the egg cell and the secondary nucleus is called double fertilization. After double fertilization, the ovule develops rapidly. The fertilized egg cell develops into the embryo, that is, a tiny plant with a radicle, hypocotyl and cotyledons; the sperm that fused with the secondary nucleus develops into the endosperm nucleus and then into the nutritive tissue of the seed (the endosperm); and the integument develops into the hard seed coat. These are the most important parts of the seed. During the process of seed maturation, the endosperm is gradually used up.
The mark left on the seed where the seed stalk falls off is called the hilum. On a mature seed a very small opening can also be seen, namely the micropyle. Under a high-power microscope the seed coat shows beautiful patterns. Some cactus seeds also bear special structures, such as an aril. It develops from the seed stalk and forms a long, strip-like protrusion on the outside of the gradually maturing seed. Some seeds also bear a spongy or corky appendage near the micropyle.
When a seed germinates, it first swells, then the seed coat cracks and the radicle emerges from it. At this time, ensuring water absorption is most critical, because the nutritive tissue of the seed can still meet the nutrient needs of the new sprout for a period of time. Because most cactus seeds germinate close to the soil surface, the green seedlings can quickly begin photosynthesis. We will explain the conditions for germination in more detail in the section on sowing. Many columnar cacti take many years to flower for the first time. As the plant grows, large tree-like or shrub-like cacti produce much corky tissue within the plant. This lignified tissue (xylem) is also produced by the cambium, eventually forming a cylinder-like protective layer. The xylem of cacti has no conspicuous annual rings, so people cannot determine their age from it. It is speculated that giant tree-like cacti, such as those of the genus Carnegiea, can live up to 250 years. The xylem of cacti is mainly composed of rectangular lignified thick-walled cells. Unlike most wood, the cells of the xylem are living and contain starch. The transport of water is mainly carried out by long tubular cells (that is, vessels).
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A Detailed Explanation of Cactus Leaves and Roots
The Formation of Young Cactus Stems
The External Structure and Morphology of Cacti
An Outline of the Evolution of the "Living Fossil" Cactus