THE DEPTH IN THE SOIL AT WHICH PLANTS OCCUR. 4Q1 



of these the effect is relatively small ; whilst in the Bramble 

 adventitious roots arise from the pendent tips of straggling 

 branches resting on the ground and draw them under the surface 

 of the soil. Next season the tip grows up again into the light, 

 and on the decay of the connecting branch we have an inde- 

 pendent rooted plant. 



Though a certain degree of contractility appears to be a 

 widely spread property in roots, it must not be regarded as a 

 universal one. Many plants possessing demonstrably contractile 

 roots have others in which the property is lacking. In other 

 cases all roots are contractile, whilst in many instances (e.g. 

 Colchicum autumnalc, Tulipa gesneriana, Zea Mays, and other 

 grasses, &c.) none of the roots of the plant show any trace of 

 contractility. Not infrequently the contractile -roots are more or 

 less fleshy (cf. figs. 113 and 114), and often are of a characteristic 

 spindle-shaped figure. Such roots, particularly amongst Mono- 

 cotyledons, owe their fleshiness to a large development of cortex 

 — of the tissue, that is, in which the contractility especially 

 resides. And it is amongst these fleshy roots that the phenome- 

 non is best exhibited, though even amongst these contractility is 

 not always present, for Rimbach was unable to demonstrate its 

 presence in terrestrial Orchids (e.g. Orchis maculata, Listera 

 ovata). In any case such roots play an important part as reservoirs 

 of reserve materials, and in some cases the plant depends entirely 

 upon them for storage during the dormant season. That the 

 functions of storage and contractility should be so often concen- 

 trated in the same structure is readily intelligible. For the bulky 

 roots adequate for the first of these functions are, owing to the 

 powerfully developed cortex, most effective in the carrying out 

 f the second. 



It will now be convenient to illustrate the foregoing more 

 general remarks by reference to special cases, and for this 

 purpose the life-histories of a limited number of types may 

 be epitomised. 



(1) All roots contractile, the primary root not persisting. 



Amongst Monocotyledons the phenomenon has been carefully 

 followed by Rimbach in numerous bulb- and corm-possessing 

 plants belonging to the Liliaceas and Amaryllidaceae. 



Phadranassa chloracea (fig. 114). — The seed germinates at 

 the surface, and forthwith the plumule and radicle are not only 

 pushed out, clear of the seed, by the elongation of the sheathing 



»2 



