Core Practical 7: Observe and Draw Plant Structures — Complete Guide
Edexcel IAL Biology A Level的Core Practical 7（核心实验7）要求你用光学显微镜观察、绘制并标注植物根、茎、叶的横切面结构。这不仅是一个动手实验，更是考试中实验设计类题目的高频素材。掌握这个实验，你将在Paper 3和Unit 3中占据先机。
Edexcel IAL Biology A Level’s Core Practical 7 requires you to use a light microscope to observe, draw, and label transverse sections of roots, stems, and leaves. This isn’t just a hands-on lab — it’s high-frequency material for experimental design questions in your exam. Master this practical, and you’ll have a clear edge in Paper 3 and Unit 3.
🔬 5 Essential Concepts Behind Core Practical 7
1. Plant Tissue Types You Must Identify (必辨别的植物组织类型)
实验要求你识别三类关键组织：Xylem vessels（木质部导管）——厚壁、中空、被木质素染成红色的管状结构，负责水分运输；Phloem sieve tubes（韧皮部筛管）——薄壁、较小、运输有机养料；Sclerenchyma fibres（厚壁组织纤维）——极为厚壁、提供结构支撑。考试常问：”How can you distinguish xylem from phloem under a light microscope?”
You must identify three key tissue types: Xylem vessels — thick-walled, hollow, lignified (stains red), responsible for water transport; Phloem sieve tubes — thin-walled, smaller, transport organic solutes; Sclerenchyma fibres — extremely thick-walled, provide structural support. Exam question: “How can you distinguish xylem from phloem under a light microscope?”
2. The Staining Technique (染色技术要点)
本实验使用 Toluidine Blue O（甲苯胺蓝O） 染色剂。关键步骤：滴加2-3滴染液后等待3分钟充分渗透，然后用吸水纸吸除多余液体。Toluidine blue能将木质化的xylem染成蓝绿色，将薄壁组织染成紫色，实现组织对比。染色不足会导致结构模糊、无法准确识别。
This practical uses Toluidine Blue O stain. Critical steps: add 2-3 drops and wait 3 full minutes for penetration, then blot excess with paper towel. Toluidine blue stains lignified xylem blue-green and parenchyma purple — creating the contrast needed for tissue identification. Under-staining leads to blurry structures and misidentification.
3. Sectioning Technique — Getting the Thinnest Slice (切片技巧)
这是整个实验中最需要手动技巧的环节。用剃须刀片沿着茎的横截面方向轻轻滑动（不是下压切割），多次尝试后选取最薄的标本放入培养皿中。标本太厚会导致光线无法穿透、细胞层叠、结构无法辨认。考试评估标准会考察你是否理解”thin section”对光学显微镜观察的重要性。
This is the most technique-sensitive part of the practical. Use a razor blade to glide lightly across the stem’s cross-section (don’t press down), make multiple attempts, and select the thinnest specimen for the slide. Sections that are too thick prevent light penetration, cause cell overlap, and make structure identification impossible. The mark scheme will test your understanding of why thin sections are essential for light microscopy.
4. Biological Drawing Standards (生物绘图标准)
Edexcel对生物绘图有严格的评分标准：使用铅笔（不可用钢笔）、清晰的连续线条（不涂鸦、不阴影）、正确的比例、标注线必须用直尺画出并精确指向结构、标注写在图外。绘图时标明 magnification（放大倍数），并注明你所用的物镜倍数（低倍或中倍）。
Edexcel has strict marking criteria for biological drawings: use a pencil (not pen), clear continuous lines (no sketching or shading), correct proportions, label lines drawn with a ruler and pointing precisely to the structure, labels written outside the drawing. Always state the magnification and note which objective lens you used (low or medium power).
5. Root vs Stem vs Leaf — Key Structural Differences (根茎叶结构对比)
考试中最常见的比较题：Root（根）——维管束在中心，有内皮层和凯氏带，无叶绿体；Stem（茎）——维管束排列成环状，xylem在内、phloem在外，皮层较厚；Leaf（叶）——有上下表皮、栅栏组织和海绵组织区分，气孔主要在下表皮。这种对比题几乎是Unit 3的必考内容。
The most common comparison question in exams: Root — vascular bundle in the center, has endodermis with Casparian strip, no chloroplasts; Stem — vascular bundles arranged in a ring, xylem inside and phloem outside, thick cortex; Leaf — upper and lower epidermis, palisade vs spongy mesophyll differentiation, stomata mainly on lower epidermis. This comparison is almost guaranteed to appear in Unit 3.
📚 Study & Exam Tips
- 动手练习绘图：考试中的绘图题占分不低。至少练习绘制3次根茎叶横切面图，确保比例和标注准确。Practice your biological drawings at least 3 times — marks are awarded for accuracy and completeness.
- 掌握实验误差分析：常考的系统误差来源——切片太厚（光线无法穿透）、染色时间不足（对比度不够）、盖玻片下有气泡（干扰观察）。Know the common sources of error and how to improve.
- 联结理论与实验：Core Practical 7与transport in plants（植物运输）章节紧密关联。理解xylem的cohesion-tension theory和phloem的mass flow hypothesis后，实验中看到的结构才有意义。Link the practical to theory — structures make sense when you understand their function.
- 记住安全要点：实验中需佩戴护目镜和手套（Toluidine blue为刺激性染色剂），使用剃须刀片时注意安全。These safety points can appear in exam questions.
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