Problem 151 - Objective Biology for NEET

Question

In cycas (a) The stems are unbranched. (b) The pinnately compound leaves persist for years. (c) Have symbiotic association with \(\mathrm{n}_{2}\) fixing cynobacteria. (d) All are correct.

Step-by-Step Solution

Answer

All the statements A, B, C, and D are correct. In particular, D which refers to all other statements being correct is the inclusively correct one.

Step 1: Verify Statement A

First check the first statement. Cycas has unbranched stems. This is a known botanic feature of Cycas. Therefore, statement A is correct.

Step 2: Verify Statement B

Now verify statement B. It is also true that the pinnately compound leaves of cycas persist for many years. This feature contributes to their survival in a variety of environmental conditions. Hence, statement B is also correct.

Step 3: Verify Statement C

Next, the third statement claims a symbiotic association with nitrogen-fixing cyanobacteria which is true. Cycas roots are often associated with certain types of cyanobacteria that help them fix nitrogen from the atmosphere. Hence, statement C is also correct.

Step 4: Verify Statement D

The final statement, D, is asserting that all the above statements (A, B, C) are correct. Since we've verified each of these statements individually and found them all to be accurate, Statement D is therefore correct.

Key Concepts

Unbranched Stems in Cycas

Cycas is one of the ancient plant species that has fascinated botanists for years due to its unique morphological characteristics. One striking feature of this plant is its unbranched stem, which grows vertically and does not typically produce side branches. This upright structure contributes to the distinct palm-like appearance of Cycas, despite it not being a true palm.

Its unbranched stem is a support column for the crown of leaves and serves multiple purposes for the plant, such as housing the reproductive structures and storing nutrients. Moreover, this growth pattern is an adaptation for survival in harsh conditions where energy conservation is crucial, as branching would require additional resources. The rigidity and robustness of the Cycas stem also help it to endure strong winds and heavy rainfall, common in the habitats where these plants thrive.

Pinnately Compound Leaves

The leaves of Cycas play a vital role in the plant's longevity and resilience. These pinnately compound leaves, composed of a central rachis with multiple leaflets arranged along it, are not only aesthetically pleasing but functionally significant. They persist for several years, which is unusual for plant leaves.

The durability and longevity of Cycas leaves are adaptations to their native environments, which can often be challenging and resource-limited. By retaining leaves for multiple years, Cycas plants conserve energy and resources that would otherwise be spent on frequent leaf production. This makes them particularly well-suited to survive in environments with nutrient-poor soil or limited water availability. Furthermore, the compound structure of the leaves enables efficient photosynthesis and water management, both critical for the plant's survival.

Symbiotic Association with Nitrogen-fixing Cyanobacteria

Another intriguing aspect of Cycas botany is the plant's symbiotic relationship with nitrogen-fixing cyanobacteria. These microscopic organisms live within specialized roots of Cycas known as coralloid roots, which have a unique, coral-like appearance.

The symbiotic cyanobacteria belong to the genus Anabaena and play a crucial role in converting atmospheric nitrogen (2) into a form that plants can use. This process, called nitrogen fixation, is vital for the growth and nutrition of Cycas, especially in soil that is low in nutrients. The relationship is mutually beneficial; the cyanobacteria procure a protected habitat and access to plant-produced carbon compounds, while Cycas gains an essential nutrient without depending solely on soil fertility.

This symbiosis is a fantastic example of co-evolution and resource-sharing in nature. Such associations are part of why Cycas has endured since the time of the dinosaurs and continue to thrive in present-day ecosystems.