Anatomy and microscopic observation of Ricinus communis seed structure

<p indent=0mm>We systematically observed the structure and distribution of vascular bundles of castor (<italic>Ricinus communis</italic>) testa, endosperm and embryo with germination, separation, paraffin sectioning technique, and microscopic observation techniques, stained with safranine and fast green, along with schiff’s reagent, respectively. (1) Micropyle at the dorsal base of the seed was not covered by the caruncle. (2) The testa consisted of three parts: episperm, endopleura and the caruncle, in which the episperm was composed of the long columnar epidermal layer, sponge parenchma and palisade parenchma from outside to inside, while the endopleura was composed of Malpighian layer, sponge parenchma and the inner integument layer, with an air chamber surrounded by the inner layer and outer layer of endopleura at the end of the caruncle, similar to that of eggs. (3) Both the episperm and endopleura were distributed with vascular bundles, and the large ones of the episperm only distributed in the spongy parenchma of the ventral raphe of the seed, which extended from the hilum to the opposite end of the caruncle; while those of endopleura were also distributed in the spongy parenchtma, and they branched step by step through the bundle band, ending at the edge of the air chamber at the end of the caruncle; those of both the episperm and endopleura were continuous through the junction point of the chalazal end to form the vascular system of the testa. (4) The caruncle was composed of epidermis and parenchyma, in which parenchyma consisted of small parenchyma cells on the outside and large parenchyma cells on the inside. In the middle and ventral part of the caruncle, there were only two vascular bundles, which constituted the vascular system of the caruncle independently. In the caruncle, caruncle channel was also found, with the inner end opening in the air chamber of the endopleura and the outer end being blind. (5) No vascular bundles were found in the endosperm, which consisted of parenchymal cells containing a large number of aleurone grains. In the embryo, the vascular bundles were collateral, which differentiated in the hypocotyle and entered the main vein of two cotyledons from both sides of the top of the hypocotyle respectively, gradually reduced from 4 to 1 in the main veins, and the branches became thinner in the cotyledon, forming the vascular system of embryo. The cotyledon epidermis and mesophyll cells contained aleurone grains, but no aleurone grains were found in the plumule, hypocotyle and radicle. Two cotyledons separated from each other on the adaxial surface, while their abaxial surfaces were connected with the endosperm through the mucilaginous layers. The hypocotyle and radicle were spatially separated from the endosperm. These results provide an anatomical basis for comprehensively understanding the structure of the seeds and studying the seed development and germination and their material transportation.