804 KEPOET — 1893. 



comparison of their forms will sliow certain distinct, albeit slight, differences, 

 sufficiently marked to make it possible to identify a plant by means of the root 

 alone. 



One of the objects of this paper was to point out those differences by means of 

 diagrams in which the respective roots of Lemna minor, tvisulca, gihha, and 

 polyrhiza were represented side by side. 



Besides a considerable variety in the length of the various root-fibres, a 

 microscopic examination of the sheaths which protect the apex shows that neither 

 are they uniform in shape, but, ranging from the comparatively blunt and straight 

 ampulla of Lemna minor to the slightly pointed sheath of Lemna gibba, they 

 become blade-like in Lemna polyrhiza, and, finally, sharply pointed and with a 

 tendency to curve in Lemna trisulcn. 



As the plant matures the sheath becomes a ruddy brown colour, and is seen 

 under the microscope to be freckled with brown blotches, probably the decaying 

 outer cells of the case. This hardening of the ampulla is a very necessary security 

 against the attacks of water insects, which feed upon the delicate root fibres, often 

 commencing at the extremity of the root and working their way upwards. 



It is interesting to speculate as to possible other uses for this rather phenomenal 

 root-cap. The functions of the root-caps of terrestrial plants are easily recognised 

 in their adaptability to the purpose of forcing a way for the fibre through soil or 

 pebbles, but here we have plants suspended in the water, and yet furnished with 

 something very similar. This difficulty has been met by the suggestion that the 

 sheath of the Lemna is not a root-cap, but really a persistent digestive pouch ; but, 

 even without this explanation, one has only to take into consideration the 

 characteristics of the habitat of the duckweed to see that the ampulla is continu- 

 ally required to do the work of any ordinary root-cap of terrestrial plants. Owing 

 to the stagnant nature of the ponds and dykes where it flourishes the plant is 

 subjected to violent alternations of drought and plenty, and in the dry season 

 myi'iads of perishing Lemnre are left high and dry on the banks. The more 

 fortunate individuals, growing where the water is deeper, are gradually let down 

 as it becomes more shallow, until at last, striking their roots on the soil at the 

 bottom, they are embedded in the mire, and there await the return of rain. 



A curious hooked appearance which is occasionally seen in the ampulla oi Lemna 

 trisulca was also represented in the diagrams. 



In the long chains formed by a number of connected fronds of Lemna trisulca 

 it will not infrequently be found that the ]-oot-fibres spring sometimes from below 

 the frond and hang downwards, and sometimes from what appears to be the 

 surface of the frond, reaching upwards. A careful observation of the tendency of 

 this submerged duckweed under certain circumstances to twist into an almost spiral 

 form led the author to the conclusion that in such cases the fronds had completely 

 revolved in their sockets, so that what had at first been underneath was now 

 uppermost, throvnng the root attached to it up to the surface. Further observa- 

 tions seemed to point to the fact that this habit is confined to cases where the 

 submerged Lemna trisulca is covered from light and air by a thick overgrowth of 

 other weeds, such as Lemna minor, with which it is often associated ; for in a pond 

 where this was not the case the uncovered chains of Leinna trisulca were lying 

 almost flat, but after being placed in a basin already containing Lemna minor 

 they also assumed an irregular spiral form in the course of a few weeks. Should 

 this change be found to be attributable to a want of air it may possibly point to a 

 respiratory function in the root-fibre. 



SATURDAY, SEPTEMBER 16. 

 The following Reports and Papers were read : — 

 1. Interim Seport on the Botanical Laboratory at Peradeniya, Ceylon. 



