Page:Popular Science Monthly Volume 79.djvu/556

552 value of the great water-balances described in the life of the plant, and one naturally asks the question as to how long an individual might survive at the expense of its great accumulated supply, and what activity it may carry on while cut off from the customary supply of soil moisture. The replies to these queries vary widely with the species considered. The tree cactus may live a year or as long as two years in the open in Arizona upon its balance. Growth and reproduction are in the main inhibited, however, by any notable depletion. Sometimes, however, the death of the main trunk of a plant leaves a living branch held high in the air, and this may bloom, but this action must be due to the special stimulation of approaching death. (Fig. 4.) The melon cactus may survive one or two seasons in the open, although when given some shade individuals have been seen to live three years, carrying on some apical growth and flower formation with the usual rhythm. The prickly pears survive, grow and carry on reproduction for even longer periods.

So far as physiological usefulness is concerned, stores of water accumulated in tubers, bulbs or thickened underground organs are far more effective than thickened aerial stems or leaves in holding a water-balance available to the plant for extended periods. Ibervillea, the "guarequi" of Sonora, has a thickened stem homologous with the "Big Boot" of California, which is a relative, and it has been cited many times to illustrate observations of an individual which is still alive although detached from a supply since 1902, and has not received any notable addition since 1901. (Fig. 5.) The corms of Brodiæa form new small corms during desiccation, which are plump with the diminished supply on hand, and this process continues until the balance reaches the vanishing point in three or four years. The observations of Professor Campbell show that plants with so little external appearance of water-storage as the