35° 



THE QUARTERLY REVIEW OF BIOLOGY 



and in the region of cell division. This 

 has been taken as evidence that they 

 entered directly at this point. Priestly 

 and Tupper Carey, on the other hand, have 

 studied the chemical composition of the 

 cell wall in the root tip and find that it 

 contains proteins and fats or fatty acids. 

 These they regard as the substances which 

 render these cells impermeable, or at least, 

 not freely permeable to water. Dissolved 

 substances may, however, enter these cells 

 and hence they are able to divide, where- 

 upon they may take up water and become 

 cells of the region of elongation. 



Recently Popesco (64) has presented 

 an extensive study in which he attempts 

 to discover just which region of the 

 root does absorb. His method consists 

 in placing the roots in solutions of dyes, 

 such as eosin, neutral red, and methylene 

 blue and noting the parts which become 

 stained. He also covered different parts 

 of the root with cacao butter and noted 

 the rate of absorption of water and of dyes. 

 Finally he impregnated the root with 

 potassium nitrate or iron sulphate by 

 allowing it to grow in a solution of one or 

 the other of these compounds and then 

 sectioned and stained it so as to detect 

 where the nitrate or sulphate was located. 

 He concludes that plants absorb water 

 from a solution just as rapidly if they have 

 no root hairs as if they are well supplied 

 with them. He finds that the root does 

 not absorb through the root cap, but that 

 the region of absorption bears a definite 

 relation to the internal structure of the 

 root. That is, it is located in the vicinity 

 of the lower ends of the conducting tubes, 

 rather than being defined by the location 

 of the superficial root hairs. It usually 

 happens, however, that these two regions 

 are somewhat the same, that is, the tubes 

 usually end in the vicinity of the region 

 of the young root hairs. 



From his work it would seem estab- 



lished that the surface of the root in the 

 region of root hairs and immediately 

 below it, that is, in the region of cell 

 elongation, is as readily permeable as are 

 the root hairs. However, it does not 

 seem that he has demonstrated that root 

 hairs are not absorptive organs. In the 

 first place it must be borne in mind that , 

 much of his work is based upon the pene- 

 tration of dyes and not of water. In the 

 second place his study shows upon his own 

 criteria that in many cases, the younger 

 root hairs especially do absorb. Further- 

 more from the results of experiments to be 

 described below, it is doubtful whether 

 he has given proper attention to the 

 effect of immersion in a solution upon the 

 growth of root hairs already formed, fre- 

 quently causing a cessation of elongation, 

 and to the production of new hairs after I 

 immersion. 



That root hairs actually do absorb salts : 

 is shown by the interesting experiments of 

 Osterhout (59). He found that when root 1 

 hairs are immersed in a calcium solution 1 

 and observed with a microscope equipped i 

 with a Nicol prism, crystals are seen to form : 

 in the root hairs. These crystals he identi- • 

 fied as being of calcium oxalate. It is ; 

 thus apparent that the calcium ions have . 

 entered the cell and combined with the : 

 oxalate ions present there in solution in i 

 the form of oxalic acid or potassium 1 

 oxalate or some other salt, and that there : 

 has resulted a precipitation of the in- ■ 

 soluble calcium oxalate in the form of : 

 crystals. 



This point is borne out by the common 

 observation that when seedlings are trans- 

 planted from a hot bed to the field, for 

 instance, it is necessary that they be 

 shielded from sun and wind for perhaps 

 24 to 36 hours in order to avoid wilting 

 and possibly the death of the seedling. 

 Inasmuch as about this length of time is 

 required for the root to develop a new com- 



