This paper critically reviews the homologies and distribution of tapet
um types in monocotyledons, in relation to their systematics. Two main
types of tapetum are widely recognised: secretory and plasmodial, alt
hough intermediate types occur, such as the ''invasive'' tapetum descr
ibed in Canna. In secretory tapeta, a layer of cells remains intact ar
ound the anther locule, whereas in the plasmodial type a multinucleate
tapetal plasmodium is formed in the anther locule by fusion of tapeta
l protoplasts. In invasive tapeta, the cell walls break down and tapet
al protoplasts invade the locule without fusing to form a plasmodium.
When examining tapetum type, it is often necessary to dissect several
developmental stages of the anthers. Secretory and plasmodial tapeta a
re both widely distributed in monocotyledons and have probably evolved
several times, although there may be some systematic significance wit
hin certain groups. Among early branching taxa, Acorus and Tofieldia h
ave secretory tapeta, whereas Araceae and Alismatales are uniformly pl
asmodial. The tapetum is most diverse within Commelinanae, with both s
ecretory and plasmodial types, and some Zingiberales have an invasive
tapetum. Lilianae (Dioscoreales, Liliales, and Asparagales) are almost
uniformly secretory.