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If you have done the Class 10 activity where a leaf turns blue-black in iodine, you have repeated, in a simplified form, a line of evidence that Julius Sachs built in the 1860s. This post explains who Sachs was, what he actually showed, and how to run and explain the starch test without the usual mistakes.
Who was Julius Sachs?
A 2015 profile by Ulrich Kutschera and František Baluška gives the outline. Sachs was born in 1832 in Breslau (now Wrocław, Poland), began as private assistant to the physiologist Jan Evangelista Purkinje, and earned his PhD in Prague in 1856. In 1868 he took the chair of botany at Würzburg, where he worked until his death in 1897.
A second paper by Kutschera, Basic versus applied research, calls him the first to bring controlled, accurate, quantitative experiments into botany, and the founder of modern plant physiology. His Handbook of Experimental Plant Physiology appeared in 1865, followed by his Textbook of Botany in 1868, according to Kutschera and Karl Niklas.
What does NCERT say he found?
The NCERT Class 11 chapter Photosynthesis in Higher Plants (2026-27 reprint) says that Sachs "provided evidence for production of glucose when plants grow", that glucose is usually stored as starch, and that his later studies showed chlorophyll sits in special bodies, later called chloroplasts. It concludes that he found the green parts of plants are where glucose is made. NCERT dates the first evidence to "about 1854".
What did Sachs himself report?
Sachs described the work in his own Text-book of Botany, translated into English and published by the Clarendon Press in 1875. In the section on starch formation in chlorophyll, he reports that:
Starch appears inside the green chlorophyll grains only after they have turned green and are in fairly strong light.
When cells that have made starch are put back in the dark, the starch is absorbed and disappears from the grains, faster at higher temperatures.
When light returns, starch forms again in the same grains.
He states that he was the first to demonstrate this disappearance and re-formation in the leaves of flowering plants, citing his own 1864 paper in the Botanische Zeitung. He also describes light passed through coloured solutions: starch formed under red, orange and yellow light much as in full sunlight, but only slightly under the blue-violet half of the spectrum.
How does the Class 10 starch test use his idea?
Activity 5.1 in the NCERT Class 10 chapter Life Processes asks you to:
Keep a plant with variegated leaves, such as money plant or croton, in a dark room for three days so that its starch is used up.
Put it in sunlight for about six hours.
Pluck a leaf, mark its green areas and trace them on paper.
Dip it in boiling water, then heat it in alcohol in a water bath.
Dip it in dilute iodine solution, rinse, and compare the colour pattern with your tracing.
Each step controls one thing. The dark period removes old starch, which is Sachs's key insight. The variegated leaf gives a built-in comparison: green and non-green parts of the same leaf received the same light. Alcohol removes the green pigment so that the iodine colour is visible. Blue-black marks starch.
Common mistakes with the starch test
Skipping or shortening the dark period. Without it, starch made earlier could show up, and the test proves nothing about the light period.
Heating alcohol over a direct flame. NCERT uses a water bath for a reason: alcohol catches fire easily.
Writing that iodine detects glucose. Iodine detects starch. Glucose is made first and stored as starch.
Concluding that light alone is enough. In a variegated leaf, the pale parts got light too but made little starch. Chlorophyll is needed as well.
For the full Class 10 chapter, see our guide to life processes for Class 10. For the history before Sachs, read about Jan Ingenhousz and the role of light.
What can a student take from Sachs?
His results came from simple, repeatable comparisons: light against dark, green against pale, one colour of light against another. Each comparison changed one condition and kept the rest the same. When you design or explain any practical, name the one thing that changes and the things that stay the same. That one sentence often earns the method mark. If a diagram of the chloroplast or the path from glucose to starch still feels hazy, the Fermi Concept Library has every concept in biology with its own lesson and its place in the map.
Before your next practical, write one line for each step of the starch test saying what it controls.
