本篇文章共分 6 段，從植物防禦機制的構成，發展歷史，到科學研究所進行的實驗項目細節與結論，說明植物葉片上的防禦機制效能。

本篇考題英文原文與對應之中文翻譯整理如下。練習作答解題時若有對語意不清楚之處，請仔細查閱對照，以提升閱讀理解能力。

How Effective Are Structural Plant Defenses? 植物的結構性防禦措施效果如何？

1. 植物防禦措施歷史悠久

   Some of the least costly defenses, in terms of energy use, available to plants are structures that make penetration by predators difficult, if not impossible. These include tough leaves, spines, and epidermal hairs on leaves, which may trap, impale, or fence out insects and discourage browsing by vertebrate herbivores, as well as hard-coated seeds. These structures may have evolved early in the history of the plants, when they might have been subject to even greater predatory pressures. Because they represent little investment, plants still retain them.

   就能量使用而言，植物可用的一些成本最低的防禦措施是使捕食者—就算不是不能—但卻也難以穿透的結構。這些結構包括堅韌的葉子、刺和葉子上的表皮毛，這些可以捕獲、刺穿或隔開昆蟲，阻止脊椎食草動物的啃食，另外還有硬殼種子也是如此。這些結構可能在植物歷史的早期就已經進行演化，當時它們受到的掠奪性壓力可能更大。由於這些方式代表的是小量投資，所以植物仍然保留著它們。

2. 種皮與繁殖機制

   Many seeds have thick, hard seed coats that provide protection from seed-eating animals. The problem with such seed defense is that the seeds need to be scarified—the hard seed needs scratching or scoring of the seed coat to weaken it—so the seedling itself can escape. If the seed is not scarified, the seedling embryo is sealed in, never to germinate. Many plants, however, have turned seed predation into a mechanism for seed dispersal, such as the transport and caching of seeds by squirrels, jays, mice, and ants.

   許多種子都有厚而硬的種皮，可以保護它們不被攝食種子的動物吃掉。這種種子保護的問題是，種子需要破皮—亦即堅硬的種子需要在種皮上刮傷或劃傷，以削弱種皮的作用—這樣幼苗才能破殼而出。如果種子沒有破皮，幼苗的胚胎就會被封在裡面，永遠不會發芽。然而，許多植物將種子捕食變成了種子傳播的機制，如松鼠、樫鳥、老鼠和螞蟻對種子的運輸和藏匿。

3. 植物防禦機制效果之研究方法

   The role of structural defenses in plants was largely presumed until experimental evidence was sought to demonstrate the effectiveness of such apparent defensive structures against grazing herbivores. Researchers investigated experimentally the effects of plant spinescence on the feeding habits of three large browsing mammalian herbivores of Africa. These were the kudu, a large African antelope attaining a female body weight of 180 kilograms; the impala, a medium-sized African antelope attaining a female body weight of 50 kg; and the Boer goat, a domestic ungulate weighing about 35 kg. The experimenters hand reared the antelope from calves to allow observation of feeding habits from very close range, 1 to 5 meters, under natural conditions, to determine biting rates the animals employed. They converted the bites to dry biomass by collecting samples of leaves and shoots of a size similar to those eaten and drying them to a constant weight. They calculated eating rate as the product of bite size (dry mass) and biting rate.

   在尋求實驗證據以證明這種明顯的防禦結構對放牧食草動物的有效性之前，這些植物結構防禦的作用大多只是推測而已。研究人員通過實驗調查了植物的刺對非洲三種大型食草哺乳動物的攝食習慣的影響。這三種動物分別是撚角羚，這是一種大型非洲羚羊，雌性體重為 180 公斤；黑斑羚，這是一種中型非洲羚羊，雌性體重為 50 公斤；以及布爾山羊，這是一種家畜，體重約 35 公斤。實驗者從羚羊幼年階段開始人工飼養羚羊，以便在自然條件下從非常近的距離（1 至 5 米）觀察進食習慣，以確定動物的咬合率。他們通過收集與所吃食物大小相似的樹葉和嫩枝樣本，並將其乾燥至恆定重量，將咬合量轉換為乾燥生物量。他們將進食率計算為咬合量（乾燥質量）與咬合率的乘積。

4. 實驗項目與觀察

   Another one of their experiments involved a detailed study to examine the influence of spinescence. They selected ten plants each of five species of trees at a height accessible to impalas outside the enclosure. The woody plants exhibited three basic types of spinescence: paired prickles or thorns situated in or close to the leaf axils; short, sharp-tipped branchlets or spines, sometimes carrying small leaves; and prickles of various kinds on leaves. Thorns were either straight and long, up to 70 millimeters, or short and sharply curved (hooked). On each tree, two branches were matched for size, shape, density of leaf cover, and ease of access to impalas. These paired branches were labeled and the thorns were removed from one of the branches in each pair. Two months later, researchers visually estimated the relative loss of foliage from browsing.

   他們的另一項實驗涉及一項詳細的研究，以檢驗刺的影響。他們從五種樹木中各選取了十株植物，其高度是黑斑羚在圍場外可以吃得到的。這些木本植物表現出三種基本類型的刺：位於葉腋或靠近葉腋的成對的芒刺或棘刺；短而尖的小枝或刺，有時攜帶小葉；以及葉子上的各種芒刺。棘刺要麼是直的和長的，可達 70 毫米，要麼是短且急劇彎曲的（鉤狀）。在每棵樹上，兩個樹枝的大小、形狀、葉子覆蓋的密度和對黑斑羚的方便程度都是匹配的。這些成對的樹枝被貼上標籤，並將每對樹枝中的一支樹枝上的棘刺拔掉。兩個月後，研究人員目測了啃食造成的葉片的相對損失。

5. 實驗結果

   Results clearly showed that thorns and spines affected the feeding behavior of the three ungulates. These structures restricted bite sizes to mostly single leaves or leaf clusters, and hooked thorns retarded biting rates. Acceptability of leaves of those plant species offering small leaf size along with prickles was lower, at least for kudu, than those of other palatable plant species. The inhibitory effect of prickles was greater for impalas and goats than for kudu, which bit off the shoot ends in spite of the prickles. For certain straight-thorned species, kudu compensated partially for their slow eating rates by spending more time gathering the leaves. Most spinescent species were similar to unarmed palatable species in their acceptability to the ungulates, even though the armed species had a higher crude protein to their foliage. Probably these spinescent species, especially species of Acacia plants, would be preferred over unarmed species but for the thorns.

   結果清楚地表明，棘刺和刺影響了這三種有蹄類動物的採食行為。這些結構將咬合力限制在大部分單葉或葉簇上，而鉤狀的棘刺會延緩咬合率。那些提供小型樹葉和芒刺的植物物種的葉子的可接受度低於其他適口的植物物種，至少對撚角羚來說是如此。芒刺對黑斑羚和山羊的抑制作用比對撚角羚的抑制作用大，儘管有芒刺，撚角羚還是會咬掉嫩芽的末端。對於某些有筆直棘刺的物種，撚角羚通過花費更多的時間採集樹葉，以便多少彌補它們緩慢的進食率。有蹄類動物對大多數帶刺物種的接受程度與對無刺的適口性物種相似，儘管帶刺的物種的葉子的粗蛋白質含量更高。可能這些帶刺的物種，特別是金合歡植物的物種，如果不是因為有棘刺，會比無刺的物種更受歡迎。

6. 實驗結論

   The main effect of these armed structural defense features is to restrict bite size, thus increasing handling time. Thorns, spines, and prickles restrict foliage losses to large herbivores. In addition, the animals may incur scar tissue in the esophagus and scratches in the mouth and throat. These experiments show that at least in some cases structural plant defenses are effective at protecting plants.

   這些武裝結構防禦特徵的主要作用是限制咬合量，從而增加處理時間。棘刺、刺和芒刺限制了大型食草動物造成的葉片損失。此外，動物可能會在食道中產生疤痕組織，在口腔和喉嚨中產生劃痕。這些實驗顯示，至少在某些情況下，植物結構性防禦措施對保護植物是有效的。