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JOUENAL OF THE ROYAL HORTICULTURAL SOCIETY. 



anatomical structure. The roots of flowering plants are provided with 

 root-caps which protect the delicate tissue of the growing apex 

 within them growing through the soil. In all Dicotyledons the 

 formative tissue is in one mass, i.e. common to the root-cap (which 

 is continually renewed from below as the outer cells wear off) and to 

 the root-end. Water, however, causes a separation, so that each 

 becomes provided with its own active tissue. The root-cap thus 

 becomes independent of the root-tip. Consequently if a water-plant, 

 say the water Dropwort (Oenanthe), be pulled out of the mud, the 

 root-caps will be left behind ; the roots then look as if truncated. It 

 is quite otherwise with land plants, unless they have been formerly 

 aquatics, as the Lesser Celandine. 



The internal structure of the root of an aquatic plant shows un- 

 mistakable degeneracy. If, for instance, the roots be compared of 

 the same amphibious species, one plant growing on land and the other 

 in water, e.g. the Bur-Marigold (Bideiis), the following details will be 

 seen. The root of the land plant has the usual well-formed tap-root 

 and rootlets, with a thick zone of cortical tissue. Several strong groups 

 of woody bundles or xylem are arranged in a circle with pith in the 

 middle ; whereas in the aquatic root large air-spaces occur in the 

 cortex and pith, while the wood is very much reduced in quantity. 

 In some much reduced aquatic plants far greater degeneration takes 

 place. Thus in Hydrocleys Humholdtii the wood is reduced to four 

 vessels and the phloem to the same number ; while in Naias and Lemna 

 there are none at all of either kind, a large lacuna occupying the centre 

 of the root. 



Pneumatophores, Aerenchyma and LacuncB of Aquatic and 

 Marsh Plants. — As the absorption of oxygen for respiration of the roots 

 is of vital importance to herbs and trees, they are provided with 

 special structures. The first are known as pneumatophores or " root- 

 knees," which come above the marshy ground to store up air in the 

 hollow iuterior. The deciduous Cypress {Taxodium disticlium) is a well- 

 known example ; but when it grows in a dry or sandy soil they are not 

 formed. Herbs also have aerenchyma, or cellular tissue wdth lacunae for 

 storing air. The pith of a rush is a familiar instance, in which the 

 individual cells are like stars, their ray-tips only uniting. 



The MaV-sh Samphire {Salicornia herhacea) of our salt-marshes has 

 a loose cellular coat over the upper part of the root. 



Degeneracy of the Sterns of Aquatic Trees and Herbs. — Experi- 

 ments upon the comparative effects of a very dry, a very moist, and a 

 normal atmosphere respectively on ordinary woody stems proved | 

 that the proportion of wood to pith increased m dry air, but j 

 decreased in moist conditions of the air. In some cases the complete I 

 cylinder of wood of a year's growth in an ordinary tree is entirely | 

 broken up into a number of isolated " strands " of woody bundles 

 resembling that of any garden annual, or a flowerstalk of a prim- 

 rose, Slc."^' 



* See Eberhart's experiments described in .1????. de.<i Sci. Nat. t. xviii. p. 61. 

 1903. 



