Oreocereus trollii, or old man of the mountain, originates from southern Bolivia and northern Argentina. It grows on high-altitude slopes with rocky, well-drained soils. Its thick, white hairs are actually modified spines (which themselves are modified leaves). These protect the cactus from frost, drying winds, and intense sunlight. Old man of the mountain is cultivated by cactus enthusiasts, who must provide it with bright light in order for the thick white wool to form.
This species has a fairly low stature, never reaching a height greater than 100 cm. This low growth means that for most of its life, it is supported by a type of wood called wide-band tracheids (WBT). In the article Wide-band tracheids are present in almost all species of Cactaceae, James Mauseth explains that WBT wood lacks fibre, allowing the wood to contract during drought. This comes at the cost of making the wood too soft to support large, columnar cacti. Until recently, it was thought that tall cacti lacked WBT wood, but Mauseth has shown that many cacti have dimorphic wood. While young, they produce WBT wood. They switch over to a fibrous wood once they grow too tall and heavy to be held up through turgor pressure alone. Its hard to tell how tall the Oreocereus trollii in today’s photo is, but James Mauseth has posted a great photo of an old man of the mountain plant in habitat that he believes has had to switch over to fibrous wood.