DEVELOPMENT OF ROOTS. 



15 



of the shoot, is at its posterior end connected with the vascular bundles of the 

 organ bearing it; it runs through the whole length of the root, and terminates in 

 the growing point, from which it, as well as the cortex, obtains the elements for 

 its further growth in length. Through this cord, and particularly through the non- 

 lignified elements of it, the formative matters, especially such as are of proteid 

 nature, are conducted to the growing point from the mother-organ. Since the 

 roots have to absorb water and the nutritive matters dissolved in it, and do not 

 need protection against evaporation in the soil, they are devoid of the resistant 

 epidermis of the shoot-axis, the outer wall of the cells is not at all or very slightly 

 cuticularised, and ■ obviously the stomata are wanting, since in roots there is no 

 question of a rapid interchange of gases with the 

 environment. 



When a new root-fibre is about to arise, there 

 is formed in the interior of the tissue of the 

 mother-organ, for instance inside a root, or a shoot- 

 axis, or even a leaf, in the first place a new grow- 

 ing point, which usually consists of small-celled 

 embryonic tissue, which becomes at once clothed on 

 the outside with the root-cap. These new growing 

 points generally arise on the outer side of the vas- 

 cular bundles of the mother-organ, and when the 

 young roots begin to grow, they must first break 

 through its cortical tissue, as may easily be observed 

 in fresh roots : the lateral roots of these protrude 

 from slits, the edges of which are often raised up 

 in a lip-like manner. As a rule, a root, and likewise 

 a shoot-axis, gradually produces a large number of 

 root-fibres, which on their part generally act in the 

 same manner. In this the rule holds good which 

 prevails generally in the formation of organs in the 

 vegetable kingdom, that the roots appear in acro- 

 petal succession — i. e. the youngest roots are always 

 nearest the growing point of the mother-root, or 

 of the root-forming shoot, and the further they 

 are removed from the growing point of the mother- 

 organ, the older and longer they are. Since the 



young roots arise on the outer side of the vascular bundles, or at any rate in 

 definite relation to the vascular bundles of the mother-organ, and these run more 

 or less parallel to one another, it follows obviously that the roots which spring from 

 an organ are arranged in longitudinal rows. This arrangement appears much clearer 

 in the branching of the root itself, than in root-forming stems ; we find here the 

 lateral roots arranged in 2, 3, 4, 5 or more longitudinal rows, or orthostichies. In 

 general, however, the arrangement of the roots on their mother-organ is not so 

 strictly regular as, for example, the arrangement of leaves on their shoot-axes. It 

 is also to be insisted upon that the young roots always appear first at some distance 

 from the advancing growing point of the mother-root or mother-shoot, even though 



Fig. 7.— Longitudinal section througli the 

 root-apex of Marsilia salvatrix. -ws tlie apical 

 cell ; xy its last segments ; o the epidermis ; 

 gf the axial vascular bundle of the root ; -wh 

 layers of the root-cap ; h the parts of the root- 

 cap extending further back on the root (highly 

 magnihed). 



