796 



THE DISTRIBUTION OF SPECIES BY OFFSHOOTS. 



{Helianthus tuberosus) the tuber-forming shoots are short; the colony is therefore 

 crowded, and only spreads slowly over a larger area. The Alpine Enchanter's 

 Nightshade (Circoia alpina; see fig. 444^) forms elongated tubers at the end of 

 shoots 6 cm. in length, about 5 of them round the mother-plant, whilst each of the 

 new plants arising from these tubers repeats this formation in the same colony. 

 Since 6 cm. is a considerable length compared with the size of the Enchanter's 

 Nightshade, the group is scattered and in a few years extends over a considerable 

 area. The tuber formation of Thladiantha dubia, a gourd-like plant gi'owing in 



Fig. 444.-Plant9 with tubers and bulbs whose mode of growth leads to the formation of colonies arranged in lines and 

 clusters, i Musmri racemosum. a Thladiantha dubia. s Circcea alpina. 



Eastern Asia, is especially luxuriant; its shoot-formation is shown in fig. 444 \ In 

 this plant a whole series of tubers which are chained together by thin threads 

 4-8 cm. long is formed on each shoot. Usually they form series of 6-10, and such 

 a chain is about 50 cm. long. As a new plant grows from each tuber and again 

 produces chains of underground tubers, the Thladiantha in a few years may occupy 

 an area of 10 sq. metres, and will form a cluster which is both crowded and rapidly 

 mcreasing in circumference. A further excellent example of the same thing is 

 Glaux maritima which often spreads in the most diagrammatic manner in bare 

 sandy places near the sea-shore. 



Many tuber-forming plants producing clustered colonies inhabit marshes, espe- 

 cially such as are liable to great alteration in the level of the water and are 

 exposed in years of drought to the danger of temporarily drying up. Many Pond- 



