766 DEFINITION OF THE ROOT. 



structures, and in a floating water fern (Salvinia natans, cf. vol. ii. fig. 380) the 

 submerged leaves have the greatest resemblance to roots in shape and colour. In 

 such cases, though we can say that the leaves are metamorphosed into absorbent 

 organs, we cannot assert that they have become roots. This applies also to plants 

 whose underground stems are provided with absorbent cells {e.g. Bartsia, Epi- 

 pogium, Corallorrhiza), or whose stem-structures, submerged in water, are fur- 

 nished with epidermal cells functioning as root-hairs (e.g. Lemna trisulca). In 

 these plants the stem-structures are indeed metamorphosed into absorbent organs, 

 but they are never transformed into roots. 



We are accustomed to think of the roots of plants as organs with white, 

 yellow, red, brown, or black, but never green, colour, because as a matter of fact 

 by far the greater number are devoid of chlorophyll. But there are plants whose 

 roots do contain chlorophyll, e.g. those of Lemna minor, and various aroids and 

 orchids. In orchids with aerial roots but no green foliage-leaves, the green roots 

 must take on the formation of organic compounds from food-gases in sunlight, 

 that is, the function which is performed by the foliage-leaves in so many other 

 cases. We should, therefore, be as little justified in bringing forward the absence 

 of chlorophyll as a characteristic feature of roots as in saying that the roots had 

 become changed into green leaves. The roots of the orchids mentioned have indeed 

 become transformed into assimilating organs, but they remain roots nevertheless. 



It was formerly thought that roots and stems could be distinguished, the former 

 by their inability to develop buds, and the latter by their power of forming them. 

 But although this difference is actually observed in most instances, it cannot be 

 applied universally. The roots in many plants develop buds which unfold into 

 leafy shoots, and not merely lateral, but terminal buds also. When this happens 

 it looks as if the root were continued directly into a leafy shoot, and this occur- 

 rence has led to the mistaken idea that the root-tip may become metamorphosed 

 into a leafy stem. 



Finally we have to consider the difierence in the mode of origin of roots and 

 stems. It cannot be denied that the points of origin of stem-structures are usually 

 arranged geometrically, while roots only exhibit such an arrangement in rare 

 cases. But we must again insert the words "usually" and "rare", for here too 

 a universal distinction does not exist. The stem-structures springing from the 

 underground roots of the Aspen (Populus tremula), and from old trunks of the 

 Black Poplar (Populus nigra) make their appearance quite irregularly, whilst, 

 on the other hand, the roots of many aroids originate with the same regularity 

 as leaves and the lateral shoots arising from the axils. In most cases the root 

 proceeds from a group of cells in the interior of a stem or older root, and it used 

 to be thought that this constituted a difference between roots, and stems and leaves, 

 since the latter arise from cells near the surface of the tissue- body which bears 

 them. But aquatic roots, e.g. those of Ruppia and Zannichellia, also proceed from 

 cells near the surface of the stem, and in the same way roots arise from the epi- 

 dermal cells of the leaves of the Cuckoo Flower (Cardamine pratensis), and from 



