Cacti and other succulent plants originate in areas where water is only occasionally available and are, therefore, conditioned to deal with long periods of drought. They possess structural modifications enabling them to store moisture for use in times of scarcity.
Such adaptations may be similar in both groups. (All cacti are succu¬lents but not all succulents are cacti.) Storage areas include thickened leaves, stems, and corms. Leaves, which transpire precious moisture, may be eliminated altogether (with the stem taking over the process of photo¬synthesis), or the moisture in the leaves may be protected from evapora¬tion by a leathery surface or covered with wiry or velvety hairs, thick spines, or even with a powdery coating.
The very shape of many succulents provides the same protection; globular and columnar forms offer the least exposed area to the drying effects of sun and wind.
Many times there are “look-alikes” in the two groups. Certain cacti coming from the New World closely resemble counterparts in the Euphor¬bias of Africa.
How do we then differentiate between cacti and other succulents? It is not always easy. Presence or absence of leaves can be helpful; size and Brilliance of flowers are also helpful, but the real test comes by learning to recognize the areole.
The areole is possessed by cacti alone and consists of cushion-like modifications on the body of the cactus from which arise spines, hairs (and the barbed hairs or spines of Opuntia), flowers, fruit, and often new growth.
The flowers of cacti are usually more conspicuous and most often appear from areoles near the top of the plant. In other succulents they are inclined to be less showy and more likely to emerge from between the leaves or from the base.
In addition, with a very minor possible exception (a form of Rhipsalis), all cacti are native to the Western Hemisphere. It is sometimes hard to believe this because of the vast areas of escaped cacti in many parts of the world today.
The majority of other succulents (excluding Agave, Echeveria, Sedum, Sempervivum and a few others) are indigenous to Africa and a few scat¬tered areas in the Eastern Hemisphere.
Both cacti and other succulents are excellent subjects for the outdoor garden, greenhouse, or windowsill. They require a minimum of care, pro¬vided that they have a requisite amount of sunlight and that their condition of hardiness is respected.
(A) Succulents and non-Succulents
(B) Regions of the World and their Vegetation
(C) Distinguishing Between Succulents and Cacti
(D) Subjects for the Outdoor Garden
(E) Characteristics of Cacti and Other Succulents
PASSAGE 2
The torpedo is a self-propelled underwater weapon having either a high-expl osive or a nuclear warhead. Conventional warheads are loaded with up to 1000 pounds of HBX explosive.
Underwater explosion of the torpedo warhead increases its destructive effect. When a projectile explodes, a part of its force is absorbed by the surrounding air. Upon explosion of the torpedo warhead, the water trans-fers almost the full force of the explosion to the hull of the target ship.
Fleet-type and Guppy submarines are fitted with 10 tubes, 6 in the bow and 4 in the stem. Spare torpedoes are carried in ready racks near the tubes. On war patrol, a submarine of this type usually puts to sea with a load of 28 torpedoes aboard.
Torpedoes are propelled by gas turbines or electric motors. Turbine types have maximum speeds of 30 to 45 knots, with a maximum effective range of as much as 7% miles. Electric torpedoes usually have less speed and range than turbine types, but from the submariners point of view, they have the advantage of leaving no visible wake.
(A) The Dynamics of Underwater Explosions
(B) The Efficacy of Conventional vs. Nuclear Warheads
(C) Standard Submarine Equipment and Armament
(D) How Torpedoes Work
(E) Torpedo Functions and Deployment
Answers and explanations
GRE Reading Practice Main ideas, topic, best title.
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