8i4 



A TEXTBOOK OF THEORETICAL BOTANY 



{b) Contractile Roots. 



A very interesting modification is shown by contractile roots, which 

 are formed by many perennial herbaceous plants. The most striking examples 

 are found on the bulbs and corms in Monocotyledons of the families Liliaceae 

 and Iridaceae (Fig. 8ii). Perennial tap roots such as those of the Dandelion 

 are also often contractile though to a lesser extent. These specialized roots 

 are long, straight, thick and fleshy. In young plants of Gladiolus they may 



FLOWERING 

 STEM 



YOUNG CORN 



OLD CORN 



CONTRACTILE 

 ROOT 



ADVENTITIOUS 

 ROOT 



Fig. 8ii. — Crocus sativtis. Corni with contractile roots. 

 {After Fritsch and Salisbury.) 



be thicker than the corm itself. They grow straight downwards, without 

 branches or root hairs. They only last for two or three months and they 

 contain at first abundant stores of glucose, which are rapidly consumed. As 

 the reserves disappear, the cortex collapses in a series of transverse zones, 

 which shortens the whole root by 30 to 40 per cent. This exercises a forceful 

 pull upon the plant. In the case of bulbs and corms, where these roots are 

 produced only at the close of the growth season, the plant is drawn bodily 

 downwards and the yearly repetition of the process carries the plant down 

 to a certain level, after which the formation of contractile roots ceases. If the 

 bulb is then dug up and replanted near the surface, however, the process 

 begins again. 



Contractile roots are also formed in some rhizomatous species such as 

 Polygonatum and Asparagus, where no drawing of the plant downwards results 



