DIV. I 



MORPHOLOGY 



177 



exposed to the danger of drying. The buds that persist through the 

 winter may be just above the surface of the soil but protected by 

 fallen leaves or by snow, or they are subterranean and more effectively 

 protected both against desiccation and frost by the surrounding earth. 



When the persisting buds are above ground they may be borne on creeping 

 surface shoots (e.g. Saxifraga, Stellaria holostea, Thymus, etc.), or are subterranean 

 shoots or rhizomes, as in the perennial rosette plants (Bellis, Taraxacum, Primula}, 

 and in biennials which pass the winter with a rosette of leaves (e.g. Verbascum, 

 Digitalis, etc.). Here 

 also, as in the case of _ ,*_ 



geophytes, subterra- ff^^jj^ 



nean storage organs ^ i-.. 



may occur. * 



In the GEO- 

 PHYTES (^ or 

 herbs with subter- 

 ranean buds which 

 persist through the 

 winter, the parts 

 which bear the buds 

 have a construction 

 corresponding to 

 their life in the soil. 

 They may be meta- 

 morphosed shoots 

 (RHIZOMES,TUBERS, 

 BULBS), or meta- 

 morphosed roots FIG. 203. Part of a growing Potato plant, Solanum tuberosum. The 

 (ROOT-TUBERS).The whole plant has been developed from the dark -coloured tuber 



buds that form new 

 shoots in the spring 

 require a supply of food materials, especially when they are placed 

 some distance below the surface. These food materials were constructed 

 in the preceding favourable season before the aerial shoots died down. 

 The subterranean organs, formed largely of storage parenchyma, are 

 naturally thick or swollen, to allow of the accumulation of reserve 

 materials. Such storage organs may be modified stems, leaves, or 

 roots. They become gradually depleted at the commencement of 

 the period favourable for vegetation, and then (except in the case of 

 many rhizomes) perish and are replaced. 



(a) Root -stocks or RHIZOMES and STEM -TUBERS are colourless 

 subterranean shoots, the former being thick or relatively thin with 

 shorter or longer internodes (Figs. 123, 138), while the latter (e.g. 

 the Potato-tuber, Fig. 203) are greatly thickened. The leaves, as is 

 the rule in subterranean shoots, are developed as scales. The reserve 

 materials are stored in the stem, which is on this account usually 



N 



n the centre. (From nature, copied from one of BAILLON'S 

 illustrations, nat. size. After SCHENCK.) 



