Question

Observing specimens of land plants under the microscope, you see structures not found in algae. What challenge of living on land does each structure address? A. gravity B. competition C. desiccation D. radiation 1. Spores and pollen are covered by a tough, yellow, substance made of cross-linked lipids. 2. The tip of the shoots and roots have an apical meristem that allows rapid growth. 3. The shoots have many colorful pigments besides those needed for photosynthesis 4. Stems have cellulose and lignin

Answer 1

Answer:

Explanation:

Gravity: One of the adaptations of land plants are the deposits of lignin and cellulose. The cellular wall is composed of two phases:

-  The fibrillar phase, which is inert, rigid and constituted by long cellulose molecules. Its function is to shape and protect the cell.   

-  The matrix, constituted by pectic and hemicellulose substances.

Other substances can compose the cellular wall. One of them is the lignin, which is one of the most important components of the secondary wall, and after the cellulose, the most abundant polymer. During lignifications, the matrix contracts because of water loss, and the spaces left by the contraction are filled with lignin. Its function is to give rigidity to the cellular wall.

Competition: The meristem is composed of non-differentiated cells, with a thin wall, abundant cytoplasm, small vacuoles, and small endoplasmic reticule. Their size and shape vary: there are small isometric cells, and long, stretched, and fusiform cells. The apical meristems are in the roots and shoots. They compose the primary plant body. During growth the meristem cells start to differentiate, allowing the plant to get bigger and higher. In the competition for light, this is very important, especially when there are too many individuals together, like in a jungle or a forest. They need to get higher to avoid being covered by other plants and to receive more radiation for photosynthesis.  

Desiccation: Spores and pollen are covered by sporodermis, a membrane that covers and protects them. On the surface, there might be a pollinic cement, viscous and lipid, that favors adhesion with a pollinator. One of the principal functions of the sporodermis is to protect the spore or pollen from dehydration.

Radiation: Pigments are helpful filters; by their absorption, they protect different tissues from injurious radiation. When exposed to different  radiation, it is expected a change in plant pigments. This change can be related to different seasons, different stages of the plan development and different tolerance levels. Changes in pigments reflect the structural and functional reorganization of the photosynthetic system and are an indispensable part of the process of plant hardening to the changing radiation conditions.