Cacti bear little resemblance to ordinary terrestrial plants. They have no leaves, and their spindle-shaped or warty bodies are vastly different from the branches of ordinary plants. However, a few cacti, besides having typical cactus flowers, also produce common young stems and leaves, such as Pereskia. This genus consists of many-branched shrub-type or small tree-type plants. The young stems are round and slightly fleshy. Generally speaking, the lateral buds of flowering plants are borne in the leaf axils, and most of them merely become small buds without further growth. But in many plants, at least some of the lateral buds grow into short, slender shoots, such as those of <span style="color:#FF8000;">apple</span> trees and pear trees. On these young shoots, the new leaves are almost all borne at a single point
. This point is remarkably similar to the areole of a cactus, because the spines of a cactus are in fact its modified leaves.
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Pictured: young cactus stem
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  In fact, some plants do have leaves modified into prickles, such as Berberis, in which transitional forms ranging from leaf-like spines to prickles have been found. Botanists call these modified leaves and buds prickles rather than spines (only bud-derived structures are called spines, such as those of Rubus). The spines of cacti are also formed by the modification of leaves, so the botanically correct term is also prickles. However, this book still refers to them as spines according to customary usage. When new shoots or flower buds emerge from an areole, it clearly demonstrates that the areole does indeed contain buds. Theoretically,
every areole is capable of producing new shoots. If the new shoot above any areole is removed, a new shoot will emerge from that areole to replace the removed one.
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  The leaves of some Pereskia species have become very small and narrow. The genus Opuntia also has leaves, which are round but soon fall off. The leaf-like basal segments of many Opuntia plants are easily mistaken for flat new leaves. In fact, when young, these cacti still produce small and feeble leaves; new stem segments emerge from the areoles on the leaf-like stem segments, while true new leaves are no longer produced.
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  The round young stems of Pereskia often give rise to variants with hump-like, angular, or ridged stems. At this point, as the plant grows, the spirally arranged areoles become increasingly obviously arranged in longitudinal rows. The areoles of cacti with angular stems are also arranged spirally during their seedling stage.
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  The amount of water stored within the body of a cactus varies greatly with the seasonal alternation between rainy and dry seasons, and the ridged structure is particularly suited to this growth pattern.
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  When water is scarce in the dry season, the ribs become thinner; when water is abundant in the rainy season, the ribs thicken, yet the overall form of the plant does not change much. In other words, without any change in the plant's surface area, the internal water storage capacity differs greatly, like the bellows of an accordion. In contrast, the stem segments of pad-shaped cacti shrink irregularly when water is scarce. Moreover, the surface area of ribbed cacti is larger relative to the plant's total volume than that of other types (all green plants need a certain surface area to receive sunlight).
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  The number of ribs can increase to a great many. For example, many Trichocereus cacti bear a very large number of ribs,
but the genus with the greatest number of ribs is Stenocactus, which can produce as many as 120 thin, cardboard-like ribs. The young stems of many climbing and epiphytic cacti are slender and long, slightly cylindrical, or bear only irregular hump-like protrusions. The thicker young stems bear ribs, and as the plant grows, these ribs eventually become thin and wing-like, which helps the plant grow healthily in a semi-shaded environment. Later, the three-winged stems eventually give rise to two-ribbed, leaf-like flat stems.
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  On many ribbed cacti, the protrusions on the ribs are more or less distributed in a hump-like pattern. If sunlight shines directly from above, the various protrusions will to some extent shade one another from the sun. Therefore, when the shape of the ribs is clearly distinguishable, one can see that the ribs are mostly arranged in a spiral. If the ribs are composed of isolated, unconnected warty protrusions, as in Rebutia or Mammillaria, then the spiral direction of the ribs is difficult to discern, and only chains of warty protrusions arranged in intersecting diagonal rows can be seen on the plant surface. As with the arrangement of leaves, the number of warty protrusions in each row is often 5, 8, 13, 21, 34... that is, each number is the sum of the previous two. This arrangement allows the warty protrusions to be distributed densely and evenly, and maximally ensures that they do not shade one another from the sun.
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  Cacti that bear large warty protrusions differ most in form from ordinary cacti. These warty protrusions superficially resemble leaves, as in Ariocarpus or Obregonia, but the warty protrusions bear an areole, and flowers can emerge from within the areole. Therefore, the warty protrusions are not leaves but part of the stem. On some small spherical or leaf-like cacti, such as Blossfeldia or Lophophora, the spindle ridges can only be identified by the arrangement of the areoles.
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  Recommended articles:
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  The External Structure and Morphology of Cacti
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  An Evolutionary Overview of the "Living Fossil" Cacti
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  Cacti Shed Their High-Priced "Outer Garment" and Are Replaced by Low-Priced Ones
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  In Summer, Small Potted Plants Are Sought After, and Cacti Rank at the Top
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Cacti share little in common with ordinary terrestrial plants. They have no leaves, and their spindle-shaped or warty bodies are quite unlike the stems and branches of ordinary plants. However, a few cacti, besides bearing typical cactus flowers, also produce familiar young stems and leaves, such as those in the genus Pereskia. This genus consists of many-branched shrubs or small trees. The young stems are round and slightly fleshy. Generally speaking, the lateral buds of flowering plants emerge in the leaf axils, and most of them merely become small buds without further growth. But in many plants, at least some of the lateral buds grow into short young shoots, as in apple trees and pear trees. On these young shoots, the new leaves are almost all borne at a single point
. This point is remarkably similar to the cactus areole, because the spines of cacti are actually their modified leaves.
Pictured: cactus young stem
In fact, some plants do have leaves modified into prickles, such as the genus Berberis (barberry), in which transitional forms ranging from spine-like leaves to prickles have been found. Botanists call these modified leaves and buds prickles rather than spines (only structures derived from buds are called spines, as in the genus Rubus). The spines of cacti are also formed by the modification of leaves, so the botanically correct term is likewise prickle. However, in this book they are still called spines according to customary usage. When new shoots or flower buds emerge from an areole, it clearly shows that the areole does indeed contain buds. Theoretically,
every areole is capable of producing a new shoot. If the new shoot above any areole is removed, a new shoot will emerge from that areole to replace the removed one.
The leaves of some Pereskia species have become very small and narrow. The genus Opuntia also has leaves, which are round but soon fall off. The leaf-like basal segments of many Opuntia plants are easily mistaken for flat new leaves. In fact, such cacti still bear small, weak leaves when young; new stem segments emerge from the areoles on the leaf-like stem segments, while true new leaves are never produced again.
The round young stems of Pereskia often give rise to variants with hump-like, angular or ridged stems; in this case, as the plant grows, the spirally arranged areoles become increasingly distinctly aligned longitudinally. The areoles of cacti with angular stems are also arranged spirally during their seedling stage.
The amount of water stored within the body of a cactus varies greatly with the seasonal alternation of rainy and dry seasons, and the ridged structure is particularly suited to this mode of growth.
When water is scarce in the dry season, the ribs become thin; when water is abundant in the rainy season, the ribs thicken, yet the overall form of the plant does not change much. In other words, without any change in surface area, the plant's internal water storage capacity differs greatly, like the bellows of an accordion. In contrast, the stem segments of pad-like cacti shrivel irregularly when water is scarce. Moreover, the surface area of ribbed cacti is larger relative to the plant's total volume than that of other types (all green plants need a certain surface area to receive sunlight).
The number of ribs can increase to very many; for example, many cacti of the genus Trichocereus bear a great many ribs,
but the greatest number of ribs belongs to the genus Echinofossulocactus, which can produce as many as 120 thin, cardboard-like ribs. The young stems of many climbing and epiphytic cacti are slender and long, slightly cylindrical or bearing only irregular hump-like protrusions; the thicker young stems bear ribs, and as the plant grows, these ribs eventually become thin and wing-like, which helps the plant grow healthily in a semi-shaded environment; later, the three-winged stems ultimately give rise to two-ribbed, leaf-like flat stems.
In many ribbed cacti, the protrusions on the ribs are more or less distributed in a hump-like pattern. If sunlight shines directly from above, the various protrusions shade one another to a certain extent. Therefore, when the shape of the ribs is clearly discernible, one can see that the ribs are mostly arranged spirally. If the ribs consist of separate, isolated wart-like protrusions, as in the genera Rebutia or Mammillaria, then the spiral course of the ribs is difficult to discern, and on the plant surface one can only see chains of wart-like protrusions arranged in oblique, intersecting rows. Like the arrangement of leaves, the number of wart-like protrusions in each row is often 5, 8, 13, 21, 34..., that is, each number is the sum of the two preceding numbers. This arrangement allows the wart-like protrusions to be distributed tightly and evenly while maximally ensuring that they do not shade one another.
Cacti bearing large wart-like protrusions differ most in form from ordinary cacti. These wart-like protrusions superficially resemble leaves, as in the genera Ariocarpus or Obregonia, but each wart-like protrusion bears an areole, and flowers can emerge from within the areole. Therefore, the wart-like protrusion is not a leaf but part of the stem. In some small spherical or leaf-like cacti, such as the genera Blossfeldia or Lophophora, the spindle-shaped ribs can only be identified by the arrangement of the areoles.
Recommended articles:
The external structure and form of cacti
An evolutionary outline of the "living fossil" cactus
Cacti shed their high-priced "outer coating" and are replaced at low prices
Small potted plants are sought after in summer, with cacti among the top