To solve this problem, we need to analyze the given statements about cleistogamous flowers:$\newline \bullet$Statement I: Cleistogamous flowers are invariably autogamous.$\newline$ - Cleistogamous flowers are those that do not open and are self-pollinated.$\newline$ - Since they do not open, there is no opportunity for cross-pollination, making them strictly autogamous.$\newline$ - Therefore, Statement I is correct.$\newline \bullet$Statement II: Cleistogamy is disadvantageous as there is no chance for cross-pollination.$\newline$