本文首次探討臺灣本島及太平島和東沙島間之植物區系關係。植物區系乃表示兩地植物相的相似度，本文採用Sorenson（1948）計算法，由此所得太平島及東沙島之相似度（Is）為59.43%，顯示此兩島的區系關係並不算密切。當兩島(地)面積大小懸殊，難以比較時（大小相差百倍以上），以兩島（地）之共有度（Ic）表示小島相對於大島的植物區系關係。計算方法是根據兩地共有的種類,除以小島的總種數；依此法可求得太平島、東沙島相對於臺灣本島的共有度值分別為87.50%及93.10%。本文所提共有度（Ic）和動物學者Simpson（1943）所提之公式，頗有類同。若以兩島和臺灣海岸地區之墾丁國家公園比較，求得之共有度值太平島為84.09%，東沙島為85.06%。其植物區系關係具有高度類似性，由此推論，最主要原因係其植物相皆具有高度海岸植被因子所組成。

　　　　　The floristic relationship among the Islands of Taiwan proper, Taipingtao (Aba Itu Island) and Tungshatao (Pratas Island) is presented. The floristic relationship is expressed by the similarity index of the two latter floras. The relationship between Taipingtao and Tungshatao is calculated by the index of similarity (Is) of Sorenson (1948) as 59%. Thus, these two islands are not very closely related floristically. When two floras are not comparatiable in size i.e., the size of the large island is more 100 times bigger than that of the small island, the index of common plants (Ic) to express the floristic relationship of the small flora to the large one is proposed. The calculation is based on the common species of two floras divided by the total number of plants of the small island. Thus, the index (Ic) of floras of Taipingtao and Tungshatao to Taiwan proper is 88% and 93% respectively.The index (Ic) of floras of Taipingtao and Tungshatao to a small coastal area of Kenting National Park in Taiwan proper is 84% and 85%, respectively. The close floristic affinity among these islands is mainly due to their high composition of coastal elements in common.