The standard size of herbarium sheets is
(a) $11.5"\times 16.5"$
(b) $15.5"\times 16.5"$
(c) $18.5" \times 10.5"$
(d) $20.5"\times 21.5"$

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Hint: A herbarium is a tool to study biodiversity. They are a referral system for correct and authentic identification of unknown plants. A systematic procedure is followed to collect and store plant specimens for future use.

Complete answer:
A herbarium is a place where after a plant specimen has been dried and pressed, they are mounted on sheets and arranged according to any widely accepted system of classification. The standard size of sheets used is $30\times 45$ cm or $11.5" \times 16.5"$.
It serves as an information source of flora distribution in an area which can be valuable for breeding programmes and research purposes. Many botanical gardens and institutes maintain large herbaria for botanists and students. E.g. Royal botanical garden in London with 6.5 million herbarium specimens.

Additional Information:
Following are the methods involved in the construction of herbarium:
-The site for specimen collection varies according to its type and season of its growth. Tools for plant collection include cutters, diggers, sickle etc.
-The plant specimen is further collected in dry polythene bags or vasculum which are special boxes to keep the material airtight and prevent it from wilting till it is pressed on drying sheets.
-The specimens are carefully spread over flimsies (thin sheets) to have a separate and proper view of each of its parts.
-In a special instrument ‘plant press’, the specimen is pressed and left to dry. Sheets are changed at an interval of 3-5 days.
-The dried specimen is transferred and pasted onto a herbarium sheet which is available in different colours. Its standard size is $30 \times 45$ cm.
-Labels ( $7 \times 12$cm) are fixed on the lower right-hand corner of the sheets.
So, the correct answer is ‘ $11.5" \times 16.5"$.’

Note: Labels are very important while maintaining herbaria because they impart valuable information about the specimen. It includes the family, genus, species, collectors’ name, plant characteristics, area and date of collection. To protect the sheets from insect or fungal attack, they are treated with 0.1% mercuric chloride or DDT or carbon disulphide as pesticides. Soft or bulky parts like cones, seeds or fruits are stored in labelled boxes to preserve their 3-D structure.