Formation and development of rhizoids of the liverwort Marchantia polymorpha

This paper mainly studies the formation and development of the rhizoids of Marchantia polymorpha L. using light and electron microscopy, and cytochemical methods. Marchantia polymorpha has highly specialized rhizoids that can be divided into two types, namely, tuberculate rhizoids and smooth-walled rhizoids. Tuberculate rhizoids individually originate from the lower superficial cells of the apical meristem. Obvious tubercles exist in the lumen of rhizoids, which always lie parallel to the thallus surface and converge toward the midrib. Smooth-walled rhizoids always exist in clusters in free portions near the midrib in the posterior of the thallus. These smooth rhizoids always grow toward moist soil and lie perpendicular to the thallus surface. Developmental observations show that the initial cell of a tuberculate rhizoid that arises from the lower epidermis possesses a dense cytoplasm. The rapid polar growth of a rhizoid produces a non-pigmented tubular cell. A growing rhizoid possesses an apical cytoplasmic mass, which contains numerous vesicles, Golgi bodies, and peripheral endoplasmic reticula. The nucleus always follows the apical cytoplasmic mass. The remainder of the cytoplasm located behind the nucleus appears to undergo a degeneration process. Simultaneously, a tuberculate wall is formed by the deposition of fibrillar materials onto the inner side of the primary wall. Histochemical investigation shows that the wall of the rhizoid is polysaccharide in nature and most likely contains callose that accounts for their fluorescence property upon staining with Aniline Blue. The base of a smooth-walled rhizoid has small tuberculated projections; however, the wall of the anterior part of the rhizoid is always smooth.