The angiospermae : roots 



797 



and the authors have observed the root of a Broad Bean seedUng which had 

 bored through the main root of another Bean seedhng close by. 



Immediately behind the initial group there Hes a zone in which cells are 

 vacuolating but are still in rapid division. Most of these divisions are anti- 

 clinal, the concentric layers maintaining their distinctness and appearing in 

 longitudinal sections as long files of cells leading backwards from the initial 

 group. Spaces containing air soon appear between the cells. 



About 3 mm. upwards from the apex the cells enter a phase of elongation, 

 and divisions become rarer. This elongating zone is very short, only i to 

 2 mm. long, but it is here that practically all the growth in length of the root 

 takes place. It is a rapid and rhythmical process, alternating in phase with 

 the cell divisions in the zone below. The maximal periods for cell division 

 are round about midnight and midday, the maxima of elongation lie about 

 halfway between these times. 



The whole force of elongation goes into the longitudinal extension of the 

 root, as there is no complication due to leaf formation, and root extension is 

 consequently much more rapid than that of the stem, a fact which we see 

 reflected in the enormous total lengths of root which may be produced. Under 

 natural conditions it takes place in the dark, which favours cell elongation, 

 but it does not seem to be affected by light of moderate intensity, to which 

 roots are usually insensitive. Longitudinal extension may be regarded as 

 the underlying principle of root growth, in contrast to that of the stem, 

 which is superficial extension, involving leaf formation. 



Root Hairs. 



The surface layer of the young root develops from the protoderm. It has 

 no cuticle and the cells remain in active 

 division farther back from the apex than 

 those of the internal tissues. In the zone 

 of cell elongation they elongate in common 

 with other cells, and immediately above 

 this zone the outgrowth of root hairs 

 begins (Fig. 793) from which this surface 

 takes the name of the piliferous layer 

 or epiblem. Not every root produces 

 hairs. As has been pointed out above 

 they are less frequent in water plants and 

 sometimes altogether absent, but it has often 

 been observed that the roots of water plants 

 which are normally hairless will produce 

 hairs if they penetrate soil or even pure 

 quartz sand. A contact stimulus seems to 

 be indicated. Exactly the opposite, namely 

 hair production only under well-aerated conditions, is found, however, in 

 Nasturtium and some other aquatics, so that the matter is evidently not simple. 



Fig. 793. — Sinapis alba (Wliitc Mus- 

 tard). Primary root of seedling 

 with root hairs. 



