NCERT Solutions for Class 9th Science Chapter 11 Let us experiment — Activity 11.3

Book page 211 – 212 Updated on2026-09-08

Q1.

After three days, observe the surface of the bread or roti carefully using a magnifying glass. Do you notice the growth of mould?

Answer

Yes. By the third day the surface carries cottony white or grey threads, and on them tiny dark pin-heads — black in Rhizopus, greenish-black in Aspergillus. As more days pass the patch spreads and darkens.

Why the moist chamber is built the way it is: mould spores need three things together, and the chamber supplies each one.

Moisture — from the wet cotton and tissue paper. A dry spore cannot germinate; that is why you add a few drops of water if the bread begins to dry.
Warmth, 25 – 35 °C — the temperature at which the fungal enzymes work fastest. Below this the spores germinate slowly or not at all, which is exactly why we refrigerate perishable food.
Nutrients — the starch in the bread or roti, which the hyphae digest and absorb.

The chamber is kept dark and away from direct sunlight because sunlight both dries the bread and carries ultraviolet radiation that damages spores.

Check it yourself: put an identical moist chamber into a refrigerator. After three days it will show little or no mould, while the warm one is covered. Same bread, same water, same spores in the air — only the temperature differs, so temperature must be what made the difference. That single comparison is a fair test.

Q2.

Observe the mould under the microscope and draw its diagram based on your observations. Compare the diagram you have drawn with Fig. 11.8 and share your observations with your classmates.

Answer

Under the microscope, after staining with cotton blue, you see a mat of branched, thread-like hyphae with upright stalks rising from it, each stalk ending in a rounded structure packed with dark spores. Match what you have drawn against Fig. 11.8.

Feature	Rhizopus — Fig. 11.8 (a)	Aspergillus — Fig. 11.8 (b)
Body	Network of colourless branched hyphae spreading over the bread	Network of branched hyphae
Spore-bearing structure	A closed round sac (sporangium) at the tip of an upright stalk	A swollen vesicle at the tip of a long stalk, with chains of spores radiating from it
Spores	Tiny round spores packed inside the sac; released when it bursts	Tiny round spores in chains on the outside of the vesicle
Colour of the patch	White cottony mat with black pin-heads	Powdery, greenish-black or yellow-green

The two moulds of Fig. 11.8 side by side. Both keep their spores high above the surface, but Rhizopus encloses them in a closed sac while Aspergillus carries them in open chains on a swollen vesicle.

Why cotton blue is added: fungal hyphae are nearly colourless and almost invisible against a bright field. Cotton blue is a dye that binds to the chitin of the fungal wall and stains it deep blue, so the outline of the hyphae and the spores stands out sharply.

Why the spore sacs are held up on stalks: a spore lying on the bread would go nowhere. Raised 1 – 2 mm above the surface, it is in moving air, so when the sac bursts the spores are carried off — which is precisely how the mould reached your bread in the first place.

Was this helpful? Report an error