Structure and Function: Cacti Spines
Cacti are extremely diverse structurally and ecologically, and so modified as to be intimidating to many biologists. Cactus evolution has been a process of diversification. Starting from some ancestral organization of stems, leaves and roots, cacti have diversified into a multiplicity of body forms. Probably no other plant family exceeds Cactaceae in diversity of structure.
Evolutionary modification of leaf morphogenesis has been extensive in all cacti, and has resulted in great diversity of leaf types within each individual plant. All cacti produce foliage leaves (microscopically small in most) and spines (modified leaves).
Cactus spines are the modified bud scales of an axillary bud; alternatively they can be considered the modified leaves of a short-shoot. Being leaves of an axillary bud, cactus spines almost always occur in clusters, a character which distinguishes this family from all others.
Each spine consists of three regions: a basal meristem; a zone of elongation/differentiation and an apical zone of mature; and dead lignified fibers. At maturity, cactus spines lack almost all characters of leaves.
A single axillary bud usually produces several types of spines, each differing in size, shape, colour and texture, varying in a predictable sequence. The outermost spines (radial spines) are usually smaller, shorter, more delicate whereas those in the centre (central spines) are more robust and have different pigmentation. Spine colour may be important for camouflage (spines often have the colour of dry grass) or for recognition by pollinators and seed dispersers, but the basis of spine pigmentation is unknown.
Spines provide more than protection from herbivores. When abundant, they shade photosynthetic cortex from intense insolation and UV.
Spines also can provide information. Isotope measurements on tissues that are sequentially added and preserved on stem succulents, such as spines, are useful tools for reconstructing past climatic agents and documenting responses to environmental change, especially in desert regions lacking other suitable proxies for recent climate changes. This relatively new discipline is called Acanthochronology.
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Photo: Barrel Cactus spines | Credit: ©Susan Ford Collins | Locality: Fairchild Tropical Botanic Garden, Miami, Florida, US (2012)