Traveller's Gourd cactus

Wise travellers in the Desert of Desolation know to look for a species of well-hidden desert cactus in the event of disaster, for it may well save their life.
The cactus widely known as the "traveller's gourd" grows in the scree surrounding outcroppings of rock in the desert, and its pale fleshy leaves closely resemble the gravel surrounding them. The leaves of the cactus closely hug the ground, and it only flowers immediately after a desert rain, when what appears to be small pebbles burst open in a riot of yellow and purple flowers. They close again just as quickly, leaving little sign of their passing.
The true importance of the gourd lies underground. Around a foot below the surface, the long roots split into vine-like structures. Fist-sized nodules along these vines are enclosed in tough bark. Each nodule contains pulp rather resembling the interior of a watermelon, mixed with fresh pure water. A traveller who has run out of water but can find a gourd plant is able to dig up a number of the gourds and eke out a few more days of survival to find a well or oasis.
It is strictly forbidden to take all the gourds from a cactus, as it is unlikely to survive. It is not unknown for tribes to discover a dead cactus which has been recently stripped of water, hunt down the traveller responsible, and put them to death for endangering others.
The cactus has a remarkable adaptation which allows it to accumulate the water it stores. Not only does it gather water from the winter and spring rains, but it is able to drop the temperature of its leaves by using thermal exchange from deep beneath the surface. It reflects much of the heat of the sun during the day, then when night falls, it rapidly cools its leaves. This encourages dew to condense on them and trickle down to its roots, which suck up the moisture and store it for later use in dry periods.