Abstract:In this test, the endodermis and Casparin strip of vegetative organs of five species of Equisetaceae were comparatively observed by paraffin section method. The morphological characteristics and distribution of two layers of endodermis and its Casparin strip were reported for the first time, and the various types of Casparin strip and their cooperative defense mechanisms with sclerenchyma were also discussed. The results showed that: (1) there was only one Casparin strip in the rhizomes and roots of five species of horsetail, and there were two Casparin strips in the aerial stems of four species of horsetail. (2) There were three types of Casparin strip in the vegetative organs of horsetails, namely, one Casparin strip on each of the two common endodermis, one Casparin strip on one scattered endodermis, and one Casparin strip on one common endodermis. (3) The rhizomes and roots of horsetails had developed sclerenchyma or dense epidermis. (4) There were complex endodermis cells in the outer endodermis of the aerial stem of Equisetum arvense L.. It was suggested that the number of Casparin strip of horsetails can not be used as a basis for classification. Although there was only one Casparin strip in the rhizomes and roots, there were developed sclerenchyma or (and) epidermis closely connected with them in the underground part. It is speculated that sclerenchyma or (and) epidermis may have the same function as Casparin strip. The defense ability of the three types of Casparin strip from strong to weak is as follows: Casparin strip on two common endodermis > Casparin strip on one scattered endodermis > Casparin strip on one common endodermis.