Arachnoid (botany)

Arachnoid as a descriptive term in botany, refers to organs such as leaves or stems that have an external appearance similar to cobwebs from being covered with fine white hairs, usually tangled. Such material is one common cause of plants having a grey or white appearance.[1] The usages of various authors in distinguishing between "arachnoid" and a few other terms referring to hairiness, such as floccose, pubescent, tomentum, cottony, or villous, tend to be arbitrary, but as a rule the term is best reserved for hairiness lighter than a felted layer, and inclined to rub off or to be easily damaged in other ways. The arachnoid appearance is common on the leaves and stems of various sclerophyllous members of the Asteraceae, such as some thistles.

Eriocephalus africanus, showing lightly arachnoid leaves, and heavily arachnoid seed follicles.

The arachnoid leaves of this Gazania are covered with a fragile cobwebby felt

Hayworthia arachnoidea - inaccurately named the "cobweb aloe" - Its spidery appearance arises from the long denticles on its leaf margins

Cephalocereus senilis is an example of a long-lasting, robust arachnoid effect created by modified spines

Nonetheless, "cobwebbiness" is a subjective impression, and in the likes of Hayworthia arachnoidea the arachnoid impression arises from the thicket of spinescent leaf denticles that are not at all fine, tangled, or fragile. In the cactus Cephalocereus senilis, the arachnoid effect arises from long-lasting hairy spines.