Making real science accessible and interesting for all people.
Monday, June 25, 2012
Life of a Cactus Part 9: Cactus Fruit and Seed
Ripe Prickly Pear Cactus fruit.
Within a month or so after flowering, the cactus flower ripens into a mature fruit ready to eat. Some cacti produce dry fruits that simply break apart and scatter the seeds. These dry fruits are typically dull brown and shriveled up. Not much exciting about this type of fruit. A large portion of cactus species though produce a brightly colored fruit swollen with sweet juicy pulp. Often these juicy fruits are ripe for harvesting during the hottest driest part of summer. May and June in in the Sonoran Desert are extremely hot and dry. It is very common for not even a trace of rain to fall during these months and temperatures will often be in excess of 110 degrees towards the end of June. During these months however some of the most abundant cacti produce their ripe fruit. Buckhorn and Teddy Bear Cholla fruit ripens in May along with many species of Hedgehog Cacti. In June then, Prickly Pears and Saguaros produce their fruits. Depending on the species these fruits are typically red, yellow, or green. While green fruit is not extremely attractive, red and yellow fruit brightly advertise their presence to all that pass by. These brightly colored fruits are plump full of moisture in the otherwise bone dry landscape, making them even more attractive to animals in search of moisture. These fruits are also often located of the tops of cactus plants where they will be most obvious. The cactus, which is otherwise extremely conservative with water liberally gives out water to any animal that desires to brave the spines. Many cacti, such as Saguaros, have very few spines on their fruits making them even more accessible. This fruit is meant to be eaten. So to make sure the fruit is eaten, the cactus makes a sweet, water rich fruit that is easily visible and accessible. Birds and animals will often gorge themselves on these fruits. These fruits may in-fact be their only water source during the hot dry late spring and summer months. The cactus liberally shares its water with these animals, but not simply because the cactus has a sharing heart. The cactus wants something in return for its water. Each of these cactus fruits is loaded with dozens to even thousands of tiny cactus seeds. Prickly Pears and Chollas have BB sized seeds that are as hard as rocks. Other cacti such as the Saguaro have tiny black seeds. These seeds are so abundant and so small in the cactus fruit, anything eating the fruit cannot avoid also eating the seeds. While the fruit pulp is full of nutrition and water, the small seeds cannot be digested and pass through the entire digestive tract of an animal unharmed. The digestive juices within the animals stomach cannot penetrate the seed coat and actually can help the seed germinate after passing through the animal. This is exactly what the cactus wants in return from the animal that eats its fruit. The cactus wants the animal to help distribute and aid the germination of the seeds. Most often, the eater of cactus fruits and distributor of the seeds are birds. After eating the fruit and seeds these birds will rest on branches of trees where the seeds will pass out of the body and be deposited on the ground. Under the canopy of Mesquites, Palo Verdes, and Ironwoods is the ideal location for a cactus seed to germinate and grow into an adult cactus. This strategy helps ensure the survival of the next generation of cacti.