STRUCTURE OF THE ROOT 89 



aside the solid particles in its course. A study of its structure explains 

 how injury to the apex is avoided. External observation shows that 

 the conical apex itself is semi-transparent and slimy to the touch. 

 By this sliminess it readily slides past obstacles, losing occasionally 

 some superficial cells in the process. But the structure of the root-tip 

 as seen in section explains the protection better ; it is well shpwn for 

 the common Barley in Fig. 63. The actual growing point is protected 

 by a calyptra, or root-cap. The superficial pilifcrous layer as it ap- 

 proaches the apex, curves inwards, though still preserving its identity. 

 Outside it lies a cap of tissue (R.C.) } which though thick at the actual 

 tip, gradually thins off as it spreads backwards. Its superficial cells are 



Fig. 64. 

 ,4=Root-tip of Buckwheat ( x 120). B=root-tip of Pea ( x 60). /?.C = root- 

 cap. Pi/ = piliferous layer. Pert' = periblem. Pter = plerome. GM = the general 

 meristem in the Pea from which the different tissues are gradually derived. 

 (After Janczewski.) 



only loosely attached. Their walls, being gummy and swollen with 

 the water in the soil, are easily rubbed away. But the loss is made 

 good by growth and cell-division arising from the innermost cells 

 just outside the incurved piliferous layer. Internally the central 

 stele continues up to the apex, ending in a rounded dome. At that 

 point a single layer of cells intervenes between it and the root-cap. 

 This layer gives rise by successive divisions to the piliferous layer and 

 the cortex. There are thus in this type, which is common for Mono- 

 cotyledons, three definite strata at the apex of the root, and each 

 gives rise to a tissue which takes special characters as it matures. 



The stratification of the growing point of roots is not always on the plan 

 described, though that is the usual type in Monocotyledons. In Dicotyledons 

 various conditions are seen, of which two examples may be given. The most 

 common is that seen in the Sunflower and Buckwheat (Fig. 04, A), where the 

 stele is as before a distinct column ending in a definite dome. The cortex 

 is also a distinct tissue covering it, but reduced to a single layer at the extreme 

 tip. The piliferous layer has a joint origin with the root-cap, by periclinnl 



