On the Life-History of Lemna minor

Abstract

1. The sporophyte of Lemna minor cannot be definitely homologized with either a stem or a leaf, but is a shoot undifferentiated except at the basal and nodal regions. 2. The secondary root is formed from a group of hypodermal cells at the node. The epidermis, which is pushed out, persists for a considerable time as the temporary root-sheath. The root-cap while young adheres to the main body of the root, but later becomes entirely free except at the growing point. The number of cells constituting the meristem region is unusually small, sometimes being reduced to one or two. An axial strand of undifferentiated cells is the representative of the conducting system of the root. 3. Flowers are rarely formed, and when present part or all of their organs may disorganize at any stage in their development. 4. In young flowers the nucellus and stamen papilla first appear, and about the nucellus is the beginning of the carpel. The spathe appears outside of the carpel and stamen papilla. The two stamens arise from the branching of the papilla. 5. A single archesporial mass appears in each stamen. This is later divided into two, then into four masses, constituting the archesporial masses of the four loculi of the anther, which four loculi constitute one and not four sporangia. 6. The primary tapetal layer is not cut off at the first division of the archesporial cells, but after these have become separated into four regions. 7. It seems clear that the tapetum is not a morphological but a physiological layer. 8. After the microspore mother cells have become free the cells of the tapetum frequently divide, pushing into the cavity of the loculus. 9. Many microspore mother cells disintegrate and function as tapetal cells. 10. The microspores germinate within the sporangium. The generative cell remains closely applied to the wall of the spore for a time before dividing. 11. In the megasporangium the primary tapetal cell usually undergoes no further division, while the primary sporogenous cell passes directly over into the megaspore. 12. Normal embryo sac structures are developed in comparatively few cases, the rule being that disorganization stops the process at some stage. This disorganization first affects the antipodal end of the sac. In sacs which have succeeded in developing endosperm we find it next attacked; and last to succumb to unfavorable conditions is the egg apparatus. 13. In case the polar nuclei fail to fuse one or both of them may develop endosperm without fusion. 14. Owing to the above conditions there is great paucity of seeds, not enough having been found to determine the homologies of the sporophyte. 15. The winter buds are summer buds which have failed to develop sufficient air spaces and gases to float them, and hence sink. When conditions become favorable again such are developed and they again come to the surface. 16. L. minor seems to have descended from terrestrial forms, and has not succeeded in adjusting to a water environment the processes involved in developing seeds.