Botany MCQs for NEET — Practice Questions with Answers

(d) Sieve elements are a component of phloem tissue and are responsible for conduction of food in plants. A sieve tube element has peripheral layer of cytoplasm without any nucleus. The central part is occupied by a network of canals which contain fibrils of p-protein. The protein is vibratite and is believed to actively participate in the transport of nutrients. One general property of its ability to form a gel and its functions as a puncture repair substance forming a plug at any site of damage in sieve element, thus preventing loss of food materials being translocated by the phloem. Thus, it is believed to have sealing function on wounding.

(d) The yearly growth of secondary xylem is distinct in the area which experiences two seasons, one favourable (spring or rainy season) and the other unfavourable (autumn, winter and dry summer). The wood formed in a single year consists of two types of wood, spring wood and autumn wood. The spring or early wood is much wider than the autumn or late wood. It is lighter in colour and of lower density. Spring wood consists of larger and wider xylem elements. The autumn or late wood is dark coloured and of higher density. It contains compactly arranged smaller and narrower elements Which have comparatively thicker walls. In autumn wood, tracheids and fibres are more abundant than those found in the spring wood.

(b) In a woody stem, due to secondary growth, secondary xylem constitutes the bulk of the stem and is commonly called wood. The secondary xylem consists of vessels, tracheids, wood parenchyma and wood fibres. Width of secondary xylem increases continuously with the age of the plant. In older stems, the wood (or secondary xylem) gets differentiated into two zones - the outer light coloured younger sapwood and the inner darker older heartwood. The xylem vessels and tracheids of heartwood become non-functional because they get filled with resins, tannins, gums etc. produced by adjacent living parenchymatous cells and due to tyloses. Changes in the nature of sapwood occur with the death of parenchyma cells and with the completion of these changes, heartwood is formed. Thus, the conversion of sapwood into heartwood is a progressive phenomenon which continues throughout the life of the plant. Initially,both sapwood and heartwood increase in size but as a tree reaches a certain age,amount of heartwood increases continuously and the amount of sapwood remains almost constant.

(d) In grasses, buliform cells (large empty colourless cells) are present that make the leaves curl inwards to minimise water loss.

Tyloses are balloon-like formation in the cavity at tracheids or vessels in the wood of trees consisting of protoplasm intruded (i.e. ingrowth) from adjacent parenchyma cells. They plug lumen, check fungal infection and increase durability of wood. With times, wall of tyloses get thick and lignified and serve as storehouse of resin, starch, gums and tannins.

P-88, PARA-1

P-86, PARA-2

During the secondary growth of the dicot root:
1. The cells of cambium strips produced towards inner side differentiate into secondary xylem elements and those produced towards outside differentiate into secondary pholem.
2. The primary remains its original position in the center of the axis but the primary phloem is pushed towards outer side and crushed.
So, the correct answer is 'It is retained in the center of the axis'.

Trochodendron and Tetracentron lack vessel elements in their wood, a quite unusual feature in angiosperms.

As these have meristematic tissue with high cell division capabilities.