May 4, 1928: 89 years ago today

Collected on May 4, 1928, this specimen was found in by Edward H. Graham in the Sonoran Desert of Mexico.  Growing up to 60 feet tall, the Saguaro cactus (Carnegiea gigantea) is among the most iconic plants of the desert southwest.  It is found in southwest Arizona and northern Mexico. Its distribution is strongly determined by freezing temperatures.
 
“Dippy” (Diplodocus carnegii) isn’t the only species named after Andrew Carnegie. In 1908, the Saguaro cactus was also named in honor of Andrew Carnegie, who funded early research in the cactus family.
 
How do you collect a cactus and press it to an herbarium sheet, you might ask?  The answer: carefully.  Sections are taken from the plant to include identifying features such as flower and spines.  The spines are modified leaves, and unlike most plants, the stem is where photosynthesis happens. The fleshy stem (photosynthetic stem tissue) scooped out and salt can be used to help dry the specimen. The result can be quite artistic.
 
Saguaros can live upwards of 200 years old!  Recent research has used the spines to reconstruct past climate conditions, and even aspects of plant physiology, using chemicals from the atmosphere (stable carbon isotopes) at the time the spine was produced.  Because the newest spines are added to the top as the Saguaro grows in height, scientists can estimate how old each spine is.
 
This Saguaro specimen was collected during museum expedition in the spring of 1928.  Specimens from this expedition helped in the design and construction of the Arizona Desert diorama in the Hall of Botany, which opened in the fall of 1929 and is just as cool to see today.