806 THE DISTRIBUTION OF SPECIES BY OFFSHOOTS. 



sleep, the starch-grains are used up in the building of the young stem, and the 

 growing buds of these species of Lemna again rise to the surface of the water, 

 because the cells which had served as storehouses for the starch become empty, and 

 because air-spaces are formed in the new tissues. 



The same change of position during the year is also observed in the offshoots of 

 the Frogbit (HydrocJiaris Morsus-rance), which is common in still waters through 

 the whole of Europe and a great part of Asia. A.lthough this plant is abundantly 

 provided with roots, it never fixes itself by them to the slimy bottom of the pool in 

 which it lives. Throughout the summer it remains floating on the surface, spreading 

 its foliage like the Water-lilies on the top, while its roots hang below in the upper 

 layers of water. Its propagation in the summer is very rapid by the formation of 

 offshoots. These arise in the axils of foliage-leaves from the very short, erect, floating- 

 stem, and are rather long, resembling thick threads, which keep close to the surface 

 of the water, and grow in a horizontal direction. Each shoot terminates in a bud, 

 and this quickly opens, sending up green foliage to float on the water, and a bunch 

 of roots below. In a very short time the plant thus formed resembles the parent 

 which gave it origin, and is itself able to develop new shoots. Thus it comes about 

 that in a few weeks the surface of the water is covered with innumerable floating- 

 plants of Frogbit, every 10-20 being connected together by horizontal strands. The 

 pretty flowers now rise above the surface from the stronger plants. The flowering 

 is, however, of short duration, and is seldom successful, i.e. fruits with fertile seeds 

 are rarely produced. As soon as the blossoming is over and autumn apj)roaches, 

 new shoots ending- in buds appear. They are shorter than those of the spring, and 

 they sink lower on account of the greater weight of the buds they carry. The buds, 

 too, have a rather different form. They are firm, and wrapped in closely-fitting 

 scale-leaves, and they almost attain the size of a small date-seed. As soon as the 

 bud is provided with the requisite amount of starch and other reserve food-substances, 

 it becomes detached from its filamentous suj^port, and sinks down (see fig. 447) til] 

 it rests on the mud at the bottom of the pond. The plants floating above, which 

 gave rise to them, die off completely and decay. It is high time indeed to quit the 

 field above, for the surface of the water is soon covered with a sheet of ice, which 

 renders all vital activity impossible for months. When spring again arrives, and 

 the ice vanishes from the pools and ponds, new life rises up from the mud below. 

 The buds of the Frogbit which have passed the winter there become spongy, the 

 cell-cavities fill with air, and the whole structure rises to the surface (see fig. 448-^). 

 Arrived there the scale-leaves rapidly separate, green leaves expand their blades on 

 the sunlit surface, roots hang down into the water, and before long, shoots are again 

 developed as already described (fig. 448^). Obviously deviations of position and 

 sometimes considerable changes of place are brought about by the sinking and rising 

 of the buds in the water. It is observed, too, that the Frogbit is very variable in 

 regard to its position, and that sometimes a place whose surface was one j^ear 

 covered with numberless plants will in the year following present no trace of them, 

 while new colonies will have developed at a distance. 



