LINEAR AND CLUSTERED ARRANGEMENT. 



797 



weeds (Potamogeton), e.g. Potamogeton pectinatus, form a large number of small 

 tubercles on their shoots which creep horizontally through the mud; Scirpus 

 tuber osus, like the Alpine Enchanter's Nightshade, forms underground shoots 10-15 

 cm. in length, each terminating in a tuber as large as a chestnut, and since the 

 plants to which these tubers give rise themselves repeat this formation of off- 

 shoots, the diameter of the clustered colony increases about 20-30 cm. every year. 

 The Arrow-head (Sagittaria sagittifolia) also develops peculiar tubers. In the 

 autumn, offshoots whose scale-leaves terminate in a sharp point not unlike those of 

 the Couch-grass spring from the knotty stems hidden in the mud. The leaf which 

 envelops the swollen end of the offshoot has a stiff point and plays the part of an 

 earth-borer or rather of a mud-borer, since it makes a path for the offshoot which 

 may elongate as much as 25 cm. The swollen end of the offshoot, which is about 

 the size of a hazel-nut, bears a small bud with greenish, closely-folded leaves, and 

 this, together with its tuberous support, remains alive during the winter, while the 

 plant to which the offshoots owed their origin perishes. In the following spring 

 each of the small buds grows up into a new plant at the expense of the reserve- 

 . materials stored in the tuber, and now instead of the old dead plant we have a small 

 group of young independent plants rising from the mud. 



The colonies of offshoots arranged in lines and clusters, which are developed from 

 underground rhizomes and shoots, elongate horizontally, and form buds laterally 

 and at their growing point, and in the same proportion as they fork and divide in 

 front they die off behind, so that the individual sprouts become separated. To this 

 category belong several species of Dentaria, Anemone, Couch-grass (Agropyrum), 

 Mint (Mentha), Yarrow (AchiUea), Willow-herb (Epilobium), Butterbur (Petasites), 

 and the Woodruff (Asperula odorata). The length of the underground shoots 

 which form the buds in these plants is very varied, as will be clearly seen from the 

 table we insert here. 



Centimetres. 



Anemone ranunculoides 

 M onarda fistulosa . . 

 Melissa officinalis . . 

 Origanum vulgare 

 AchiUea Millefolium . 

 Equisetum arvense . . 

 Asperula taurina . . 

 Oxalis corniculata. . 

 Betonica grandiflora . 

 Tanacetum Balsamita 

 Aster salignus . . . 

 Dentaria glandulosa . 

 Carex arenaria . . . 

 Juncus arcticus . . 



^5-10 



10-15 



15-20 



These numbers do not represent the lengths of single internodes, but those oi 

 the whole annual underground shoots which may consist of many internodes. 

 For example, the year's underground shoot of the Umbellifer ^gopodium Poda- 

 graria has 8 internodes, of which the proximal one is the longest and the , 



shortest. 



