598 PHYSIOLOGY. 



gams, by means of a single terminal cell only, is quite different from that 

 of Phanerogams, yet the subsequent development of the tissues takes place 

 in much the same way. 



Development of Rootlets. This takes place in various manners, and has 

 not yet been thoroughly worked out. In all cases the central cylinder of 

 the rootlet arises from the pericambium of the parent root. The root-cap 

 is derived from the internal cortical or protecting layer of the parent root. 

 In Gymnosperms the pericambium is developed into the central cylinder 

 and the cortex of the rootlet, while in Papilionacese and Cucurbitaceae it 

 forms the central cylinder only. In Vascular Cryptogams the terminal cell 

 always originates in the inner cortical layer of the parent root, and the 

 pericambium does not play so important a part in the development of the 

 tissues as in the case of Flowering plants. 



Adventitious Roots usually, but not universally, occur in places where 

 the atmosphere is warm, stagnant, and loaded with moisture. If a ring- 

 shaped piece of bark be taken from the stem, roots are formed from above 

 the wound, but not from below a circumstance supposed to be due to 

 the accumulation of organizable matter above the wound ; but by others 

 it is considered to be owing to the absence of oxygen. Portions of willow- 

 stems decorticated as above described and grown in water will produce 

 roots below the incision if exposed to the light, and none above the 

 water j and by covering the glass with black paper, and thus preventing 

 the access of light, M. Bohm has succeeded in reversing the phenomenon. 

 The portions of the stem in the water have been found by experiment 

 to give out oxygen under the influence of light, 



Direction of Growth. The downward direction of growth of the roots, 

 as contrasted with the generally upward growth of the stem and its sub- 

 divisions, is one of the most remarkable phenomena of plant-life. In the 

 case of the root, one principal reason for the downward growth is the 

 greater amount of moisture received from that side. The root grows by 

 development just within the apex ; and the multiplication of the cells in 

 that situation is dependent on a free supply of moisture. The instances 

 of roots of trees growing in the direction of watercourses or drains illus- 

 trate this ; and when plants are grown in close glass cases their roots are 

 sometimes seen to rise above ground when the confined atmosphere is very 

 moist. We have more than once observed the roots of bulbous plants, 

 growing in water or in damp sand, coil themselves in spirals. Other 

 assigned reasons are dependent on the circumstance that the soft yielding 

 extremity of the young root penetrates the interstices of the soil, and is 

 pushed down by the dilatation and expansion of the older portions above. 

 The downward direction of the root-hairs, when present, would also 

 facilitate downward growth, and prevent the root from being pushed up. 



The action of gravitation has also been considered to have some influence 

 over the downward growth by causing the sap to accumulate in the lower 

 parts of the plant, and also the varying degrees of turgescence manifested 

 by the tissues in the different regions of the stem and of the root respec- 

 tively, pith, rind, &c., such tension being directly dependent on the acti- 

 vity of the nutritive processes in the growing tissues. The direction of 

 growth depends, according to Kraus, on the circumstance whether this 

 turgescence be uniform or unequal or more in one place than another. The 

 combined action of the causes just mentioned is supposed to account for 



