In General

• Less than 1% of desert annual wild-flower seeds germinate after a brief rainstorm, whereas a heavy rainstorm causes more than 50% pf the seeds to sprout. (The Life of the Desert, Ann and Myron Sutton)
• The seeds of ironwood, smoke tree and palo-verde grow only in arroyos. These seeds are made in such a way that they will not germinate until their coatings are scarred by torrents of water rushing down the arroyo channel after a heavy rainstorm. Even more startling is the fact that seeds of the desert plants will not sprout unless the water comes from above. They are covered with a water-soluble substance that keeps them from germinating until it is removed by certain acids that are formed as the rain leaches downward through the soil. 
• Some desert annual wild flowers are among the tiniest flowering land plants on earth. A few grow as high as three or four inches, while others never reach one inch. Because they are so small that yo have to get down on your stomach to see them, they are called belly plants by botanists.
• Cacti spines are modified leaves. Thorns are modified stems. The prickles on roses are neither leaves nor stems, but simply sharp outgrowths which may develop on any part of the plant.