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 Click to enlarge During my hikes and observations, I believe that I was able to find every life stage of the saguaro except for its earliest stage as a seedling. Through research, I confirmed that these seedlings are virtually impossible to find because they grow inconspicuously among rocks and under scrubby "nurse" trees. Saguaros can produce thousands of tiny black seeds the size of a pinhead in one bright red fruit, and as many as 40 million seeds in a lifetime, but perhaps only one will actually survive to become a fully grown cactus. This is part of the reason a "nurse" tree is so important to the life of a saguaro. The tree helps protect the cactus from the hot desert sun and from winter freezes. It also hides it from animals that might eat it, and helps it to retain moisture.  Small saguaros beneath a nurse tree. Kyle estimated the smallest to be between 12 and 15 years old. Although I did not find any seedlings, I was able to find a group of small saguaros underneath a nurse tree. I noticed that the smallest cactus was covered with so many spines that it would have been virtually impossible to touch its skin without getting poked. This may make it less likely to become a meal for desert animals. The accordion-like external ribs on this small saguaro were purple instead of green. This seemed to be true only on cactuses at this stage of life, and I began to wonder if this was due to their youth. This cactus was about two inches wide, three inches tall, and five to six inches around in circumference. Using this information, I calculated that this saguaro could be about 12 to 15 years old. As I looked around for more small saguaros under nurse trees, I realized that all the little saguaros I had seen previously had been under palo verde trees. Could it be that all nurse trees are palo verde trees? I began to look for more nurse trees. After a long search, I finally found a saguaro underneath a creosote bush. However, this was the only other type of nurse tree I found. To see if my conclusion was correct, I did some research and found out that saguaro nurse plants are usually palo verde trees, creosote bushes, and sometimes mesquite trees. |
 Click to enlarge After observing the miniature cactuses with their nurse plants, I began to look at taller cactuses without any arms. This saguaro, in my observation, has accordion-like pleats that are spread far apart and provide room for me to touch its thick, waxy, green skin. This skin helps the saguaro retain moisture. As it takes in water, its accordion pleats expand so that it can store the water within its large, sponge-like stem. This is what makes the saguaro a succulent. Succulents have special roots, stems, or leaves that soak up and store water. This cactus had taken in so much water that its pleats had practically disappeared. It had many more spiny thorns at the top where the pleats come closer together, which probably help to protect the newer growth from frost and animal damage. It had developed brown calluses due to injury from insect infestation. Some of the cactuses at this stage, which were about 10 to 15 feet tall, were still next to their nurse trees, but you could see that the trees were slowly dying. As a saguaro grows, its roots spread out as wide as the cactus is tall, eventually taking over the root system of the nurse tree. The saguaro's roots grow only a few inches below the ground, so that when it rains they can soak up as much water as possible before the water reaches underground aquifers. After looking at all of these cactuses and understanding how they store water, I began to wonder how these cactuses use this water as food and nourishment. I realize that they do not have leaves like trees, so I did not think they could use the process of photosynthesis. I decided to do a little research on the matter and see what I could come up with. It turned out that saguaros do use photosynthesis, and that their thorns are actually adapted leaves. Saguaros make their own food with soaked-up water and carbon dioxide. The water travels up into the saguaro's stem. In the stem, chlorophyll is stored. Chlorophyll makes the cactus green and takes in the energy of sunlight. With this energy, chlorophyll changes water and carbon dioxide into food. This is the process of photosynthesis. |
