30 SUBTERRANEAN SHOOTS 



subterranean portions being formed of the persistent bases of 

 successive lateral axes, whose sub-aerial portions produce 

 flowers and then die. The development of the rhizome of a 

 grass as given on page 25 illustrates the mode of formation of 

 the underground sympodia. The Woodsorrel (Oxalis acetosella) 

 affords an exception to this rule ; the axis of its rhizome is a 

 single true axis which does not emerge from the soil; the 

 shoots (flowering axes) which protrude into the air are axillary 

 branches of this horizontal subterranean stem, which bears 

 scale-leaves and compound foliage-leaveS. 



A tuber is a subterranean shoot, which consists of a short 

 swollen stem bearing small membranous scales. The tuber 

 gives off adventitious roots. The Potato-tuber is a tuberous 

 stem ; its " eyes " are buds which arise in the axils of minute 

 scale-leaves. The difference between these tubers and tuberous 

 roots is well brought out by a comparison between the Dahlia 

 and the Potato-plant. The subterranean tuberous bodies of the 

 Dahlia arise on the base of the stem, in positions which bear 

 no relation to the leaves on that stem ; they possess no leaves. 

 (They arise endogenously, and their tips are clothed with root- 

 caps.) In fact, they are adventitious roots. The tubers of 

 the Potato-plant are thickened portions of lateral stems which 

 definitely arise in the axils of leaves (fig. 48) at the base of the 

 main stem of the plant; furthermore, they bear scales, and 

 when caused to develop above the soil they produce foliage- 

 leaves. (They are exogenous in origin.) 



A conn is a subterranean shoot which consists of a short 

 thickened stem more or less invested by membranous scales. 

 The corm has relatively larger scales than a tuber. 



Life-History of the Garden-Crocus {Crocus vernus) (figs. 

 49-52). — Each corm of this plant is the swollen basal part of 

 an axis which terminates in a flower ; but the corm does not 

 develop on that axis until after the latter has blossomed. 

 Examining a plant in spring (fig. 50), shortly after the flower 

 has withered (or even whilst it is flowering), we note that there 

 is a yellowish wrinkled corm, on the upper face of which is 

 either the stump or the scar of the flowering axis of the preced- 

 ing year. This is encased in brown scales, and represents an 

 axis which we will term "axis 11." On its upper face there is 

 also inserted the axis which terminates in the recently withered 

 flower {fl). This is really a lateral branch of "axis 11.," and 



