THE ANGIOSPERMAE : ROOTS 819 



is a corky layer over each root, which covers even the apex, and the portion 

 in the air bears numerous lenticels (see p. 867). Internally they possess an 

 extremely spongy cortex, and in the cortex of some species there are also 

 special cells, or idioblasts, which are empty and have thick cuticular ribs on 

 their walls. Root hairs are not produced either by the pneumatophores 

 themselves or by the horizontal roots from which thev spring, but the 

 pneumatophores form short absorbing branches from their lower portions 

 and hairs are borne on these. Experiment has confirmed the old idea that 

 pneumatophores serve as organs of aeration for the underground root system, 

 buried as it is in tenacious mud with a low oxygen content. Diffusion into 

 the horizontal roots is, however, limited to short distances. They are also 

 centres of active respiration, and their vertical growth may be necessary to 

 place the absorbing roots at the most favourable level. In sandy soils thev are 

 much fewer or may be absent. 



The submerged roots of some swamp plants, such as Jussicua in warm 

 countries, are surrounded by a greatly enlarged cortex called aerenchyma, 

 which is formed of thin, dumb-bell shaped cells, w-ith big intercellular spaces. 

 Although this is submerged, its function is probably that of increasing the 

 absorptive surface, not for water but for dissolved oxygen, which is in low 

 supply in warm water. 



(g) Haustoria 



Parasites among the Flowering Plants make use of modified roots as a 

 means of penetrating the tissues of the host plant. In Orobamhe, the Broom- 

 rape, it is the primary root of the seedling which performs this function. It 

 has no root cap and is the only root formed by the parasite, except some 

 short exogenous outgrowths, which serve only for fresh attachments and 

 may be modified roots. In Lathraea, the Toothwort, and in the green 

 semi-parasites on grass roots, of which Euphrasia, the Eyebright, is an 

 example, it is the branches from an otherwise normal root system which 

 attack and penetrate the roots of the hosts (Fig. 817). 



The Dodder, Ciisciita, has only a temporary root, in the seedling stage. 

 The embryo has no leaves, and when the root has withered the thread-like 

 stem grows independently on the surface soil for a short while, but perishes 

 if it does not quickly find a host plant. Thereafter it tw^ines around the stem 

 of the host, like its relative. Convolvulus. Its nourishment is obtained by 

 means of numerous penetrating suckers which spring from the surface of 

 its stem nearest to the host. The resemblance of these suckers to the 

 adventitious hold-fast roots of the Ivy has led to the general belief that they 

 also are adventitious roots, specially modified. If so, they have been so far 

 modified that it is difficult to find any root character which they possess. 

 The stem of Ciiscuta has four cortical layers. From the outer two there first 

 develops a flat pad which attaches itself firmly to the epidermis of the host. 

 In the centre of this pad, and involving all four cortical layers, there grows out 

 a penetrating organ, the point of which is formed of elongated cells, which 

 penetrates between the epidermal cells into the host cortex (Fig. 818). Here 



