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Each monthly issue of the Henry Shaw Cactus Digest includes club updates, columns and articles by members on their favorite aspects of cactus and succulent culture. Follow the link below this item to read select Digest articles -- or join HSCS to receive every article in the print version of the Digest.

Sticky Solutions

By Joe Merkelbach
 
Our group has a fascination with plants that have many unusual traits. I am working on a program about how convergent evolution has shaped the many similarities they share. Shapes, thorniness and limited leaves are all examples, but the fluids of cacti and succulents are a major difference.
 
Euphorbias, native to both the New and Old Worlds, have a milky colloidal fluid commonly referred to as latex. It resembles the juice of the para rubber tree but is very frequently quite toxic. Some HSCS members have tales of painful exposures to this juice. It can cause severe pain and swelling requiring medical treatment if it contacts the eyes, and is capable of causing skin rashes due to simple handling.
 
The name latex is well earned, as the para rubber tree of the Amazon River basin, Hevea brasiliensis, is a member of the Euphorbiaceae family. That juice, the source of natural rubber, is derived from tapping the outermost layer of the tree.
 
The latex source layer is not a part of the fluid system that the plants use to grow and prosper. Latex fluid is a protective feature, ranging from bad taste to extreme toxicity, that protects the plants from being eaten. The scientific view is that amino acids formed into proteins are the source of the effect.
 
Some African tribes use euphorbia latex to poison streams to gather fish, and have tipped arrows with fluid in the past. Euphorbia latex is indeed nasty stuff.
 
Cacti, on the other hand, do not use fluid as a defense. Their juice is frequently cited as a source of fluid in arid habitats, once the defense of spines is breached.
 
Pads from prickly pear opuntias, known as nopales, have been used as a food source by native American peoples for thousands of years. The juice squeezed from them is for sale as a tonic and health supplement, albeit with undocumented utility. The fruits, called tuna, are also a source of food for jams and jellies and as a base for liquor.
 
Many other species, including famously the giant saguaro, produce fruits that are edible and have a history of use by native Americans. The only chemical use of cacti widely known is that of the peyote of Mexico and Texas for its hallucinogenic properties, but that is a subject for another time.
 
This variance in chemistry is a very good indicator that even though cacti and euphorbias have many similarities, the features developed due to convergent evolution rather than showing close botanical relationships.
 
Sources:
http://forums.gardenweb.com/forums/load/cacti/msg0618271414150.html
http://www.botanical-online.com/familiacactaceasangles.htm
http://en.wikipedia.org/wiki/Euphorbia
http://en.wikipedia.org/wiki/Para_rubber_tree
http://en.wikipedia.org/wiki/Opuntia

 
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