DIV. 



, MORPHOLOGY 



167 



this is the case, for example, in the stems of Papyrus, Potamogeton,, etc., 

 in the petioles of the Nymphaeaceae, and in the roots of Jussieua. 

 Such tissue is termed AEREXCHYMA. Since its wide air-passages 

 serve for the storage of air, and allow of ready diffusion of gases 

 within the body of the plant, the rapid transport of oxygen from 

 the assimilating green organs to the colourless organs greatly 

 facilitates respiration. 



In some swamp plants, the subterranean organs of which are in swampy soil with 

 little oxygen, special organs are concerned with obtaining this gas ; respiratory 



roots (PNEUMATOPHORES, 



Fig. 188) grow erect from 

 the muddy soil, obtaining 

 oxygen from the air by len- 

 ticel - like PNEUMATHODES, 

 and conducting it by the 

 aerenchyma to the subter- 

 ranean parts. Such plants 

 are found among the Palms 

 and in the Mangroves of 

 tropical coasts, some of 

 which are also anchored to 

 the mud by a system of 

 aerial stilt-roots springing 

 from the shoots (Fig. 

 189) ( 84 ). 



2. Land Plants. 



These usually obtain 

 water and nutrient salts 

 from the soil, and 

 oxygen and carbon 

 dioxide from the atmo- 

 sphere ; their aerial 



shoots give off water in the form of vapour in the process of 

 transpiration. 



A few plants of very moist habitats, especially the Hymenophyllaceae of 

 tropical forests, which can absorb water by the general surface, form an exception. 

 Some of them develop no roots but have a system of water-absorbing hairs on 

 their stems or leaves which considerably increase the absorbent surface. 



The construction of land plants differs according to their occur- 

 rence in constantly moist localities, dry localities or climates, or 

 intermittently moist climates. 



(a) Adaptations to constantly moist Habitats. Hygrophytes ( S6 ). 



Terrestrial plants which inhabit situations in which the atmosphere 



is permanently moist, such as many tropical shade plants, are spoken 



of as HYGROPHILOUS or HYGROPHYTES. Like water plants they have 



no need of arrangements to diminish transpiration but, on the 



FIG. 188. Respiratory roots of Sonneratia alba. (Reduced 

 from a figure in Vegetationsbtidern by JOH. SCHMIDT.) 



