Page 22 - Biology - XII

Q5. Defi ne the term microsporogenesis.
Ans. The formation of microspores from the anther is called microsporogenesis.

Q6. Name the substance that makes exine and intine.
Ans. (i) Exine - Sporpollenin (ii) Intine - Pectocellulose

Q7. Can all pollens produced by an anther germinate?
Ans. No, only viable pollens germinate.
Q8. Do the fl owers (stigma) also have a nutritive medium that helps the pollen to grow on it after
pollination?
Ans. Yes

Q9. Why is nutritive medium needed?
Ans. It is needed for source of energy for the pollen tube to grow.

EXPERIMENT - 5

Objective
To make a temporary acetocarmine stained slide of onion root tip to study mitosis.

Principle/Theory
For entities to mature, grow, maintain tissues, repair and synthesise new cells, cell division is required
which happens by two types: Mitosis and Meiosis.
In mitosis, the nucleus of the eukaryotic cells divides into two, subsequently resulting in the splitting of the
parents cells into two daughter cells. Hence, every cell division involves two chief stages

• Cytokinesis—Cytoplasm division
• Karyokinesis—Nucleus division
The roots are chosen as roots are easy to grow in large numbers. The cells at the root tips are highly
dividing, hence many stages of mitosis can be seen. The chromosomes can be stained to make them easily
observable. There are three regions near the tip of an onion:
(i) The root cap containing cells that cover and protect the underlying growth region.
(ii) The meristem where cells are actively dividing in number, but not signifi cantly in size.

(iii) The region of elongation where cells are increasing in size and diff erentiating, but not increasing
in number.

Acetocarmine combines with nuclei of the cells and take a deep voilet colour allowing easy diff erentiation
for chromosomes in diff erent stages of mitosis.

Materials Required
Fresh or preserved onion root tips, needles, brush, slide, coverslip, fi lter-papers, blade, microscope, spirit
lamp, matchbox, dropper, and chemicals-acetocarmine

Procedure
To make a temporary slide/mount
1. Take a clean slide. Take the root tip and place it on the slide. Cut the root tip which is slightly paler than
the rest translucent white root.

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