There are several resources and materials that can be referred to while solving previous years' papers for the UPSC Botany optional exam. These resources can help candidates understand the exam pattern, gain knowledge about important topics, and practice solving questions to improve their performance. Here are some specific resources and materials that can be useful:

1. Previous Years' Question Papers: The first and most important resource is the previous years' question papers. These papers give candidates an idea about the type of questions asked, the difficulty level, and the overall pattern of the exam. Solving these papers can help candidates identify their strengths and weaknesses and focus on the areas that require more attention.

2. Botany Optional Books: There are several books available in the market that cover the entire syllabus of the UPSC Botany optional exam. Candidates can refer to these books to gain in-depth knowledge about various topics and concepts. Some popular books for Botany optional include "Plant Physiology" by Taiz and Zeiger, "Plant Anatomy" by Esau, and "Molecular Biology of the Cell" by Alberts et al.

3. Study Materials from Coaching Institutes: Many coaching institutes provide study materials specifically designed for the UPSC Botany optional exam. These study materials are comprehensive and cover all the important topics in a structured manner. Candidates can refer to these materials to supplement their preparation and gain a better understanding of the subject.

4. Online Resources: There are several online platforms and websites that provide study materials, notes, and practice questions for the UPSC Botany optional exam. EduRev is one such platform that offers a wide range of resources specifically designed for UPSC aspirants. Candidates can access these resources for free and use them to enhance their preparation.

5. Botany Optional Test Series: Test series are an excellent way to assess one's preparation and improve problem-solving skills. Many coaching institutes and online platforms offer test series specifically for the UPSC Botany optional exam. Candidates can enroll in these test series and practice solving questions under exam-like conditions. This will help them identify their strengths and weaknesses and work on them accordingly.

In conclusion, while preparing for the UPSC Botany optional exam, candidates should refer to previous years' question papers, standard Botany books, study materials from coaching institutes, online resources like EduRev, and participate in test series. These resources will provide comprehensive coverage of the syllabus and help candidates practice and analyze their performance effectively.