Succulent plants are odd and beautiful , and I can’t resist them. Today in a big box store attending to home plumbing repairs, the errand became more fun distracted by the purchase of two non-native succulent charmers, Lithops and Fenestraria presently sitting on the kitchen counter.
By “succulent” I’m referring to plants with thick jello-filled stems or leaves. Succulence has evolved many times in many places in many unrelated families. Today’s focus is on leaf-succulents.
Most succulents occupy sunny dry habitats. That tempts us into the obvious interpretation of succulence as an adaptation for water storage, like the hump(s) on the camel. Makes sense—if you go to the desert better have a canteen. But, as with many simplistic assumptions, a truthful reply is, “well sorta, but not exactly.” Not that I claim to have total answers! This discussion is established plant physiology mixed with an effort to weave it into a local narrative, sort of like a historian connecting the dots with fragmentary data. You are warned. These are musings, not hard fact.
So let’s get busy. First of all, why does a plant need to take in water? 1. Photosynthesis requires some. 2. Cells need some for basic maintenance, for nutrient transport, and to remain pressured so they don’t wilt. 3. Evaporative cooling. Evaporation (or to speak botanically, transpiration) is the big user, reportedly accounting for as much as 97 percent of a plant’s water uptake. Some trees transpire over 100 gallons per day bringing nutrients upward and cooling the excessive sun impinging on the foliage.
In a hot sunny dry habitat, a plant can’t afford to lavish non-existent water in massive quantities on the evaporative cooling that hot sun demands. And to make the water crisis worse, many arid plants have C4 photosynthesis, something not to explore today beyond saying it shuts down transpiration even further. (As a local example, the somewhat succulent tree Rose Apple can have C4 photosynthesis.) So you see, plants in dry hot circumstances are in a living hell! With curtailed evaporative cooling, they need a “plan B.”
Plan B is two-pronged: 1) Heat tolerance, and 2) Buffering from spikes in temperature, especially lulls in the cooling wind. Thick watery leaves resist temperature change. Think of it this way, if you pour a gallon of water out into a thin layer (representing a normal thin flat leaf) in the hot sun the water warms instantly. But if you keep that water in a jug, or in a succulent leaf, the temperature resists change. Think how long it takes to bring a gallon pot of water to boil.
Succulent leaves are not simple water bags, as anyone who has ever sliced one can attest. They don’t generally drip when cut, and their volumes don’t seem to change much relative to wet and dry weather. They are made of cells, and the cell contents are more of a gel than a liquid. Think of the gel in a disposable diaper, wet and not prone to give it up.
In Palm Beach County Florida most native succulent plants live by the sea: Saltwort (Batis maritima), Sea-Purslane (Sesuvium portulacastrum), Marsh-Elder (Iva imbricata) , Sea-Rocket (Cakile lanceolata), and more. Is it tough to take up water in maritime circumstances? Sure, rocky and sandy soils, and more importantly, salty habitats are “physiologically dry.” And to pour salt on the wound, if a plant takes up large quantities of salty water for transpiration, remember that 97%, its plumbing might choke on crud. So no surprise seaside plants are often succulent.
Often, but not always: those able to shed salt don’t all need succulence. For example, Crested Saltbush (Atriplex pentandra) disposes of the salt into hairs shaped like lightbulbs. When the hair fills with salt it bursts, and goodbye salt. This species is not particularly succulent.
Those unable to dispose of salt externally, by contrast, hold it in, and those are the seashore succulents. They store salt compartmentalized in each cell, within a large water balloon filled with salt water (technically the vacuole), like the waste collection system in an RV. A saltwater bag in each cell, drawing in ever-more water by osmosis, swells and forces bloated water-retention, suggesting why the beach is rich in thick-leaved species. The plant sheds the excess salt when it sheds the loaded leaves, like that bloated disposable diaper. Beach-succulents cultivated under salt-free conditions experimentally have shown diminished succulence.