320 SOME SPECIAL ADAPTATIONS 



Onion, and Hyacinth, the food substances are stored in 

 the thickened leaf bases. Turnips, radishes, dahlias, 

 etc., store their food substances in their roots which are 

 accordingly much thickened. Other plants develop 

 the ends of their rootstocks into storage structures, as 

 the tubers of the potato and Jerusalem Artichoke; while 

 again some thick leaves, as those of the Century Plant 

 (Agave), and many other Monocotyledons, are storage 

 organs. 



584. Habitat. Most flowering plants grow with their 

 roots in moist (not wet) soil, with their leaves in air of 

 moderate humidity. Stated otherwise we may say that 

 under these conditions the great majority of flowering 

 plants developed the forms which they have. So when 

 we say that such plants are "mesophytes" we are merely 

 stating the fact that the majority of plants live under 

 these quite similar conditions. And these have the usual 

 leaves and stems. A much smaller number have been 

 able to live in drier soil and drier air, their leaf surfaces 

 being smaller or wanting, their epidermis thicker, their 

 tissues harder, and these we have' denominated "xero- 

 phytes," literally, dry plants. On the other hand some 

 plants have been able to live partly or wholly in the 

 water. Their stems and leaves are weak and soft and 

 their submerged leaves reduced (dissected). Such plants 

 we have called " hydrophytes" (i.e. water plants). 

 Other adaptations still less marked have been noticed, as 

 the "halophytes" of salt waters or soils, the "ruderal 

 plants" of waste places, ''shade plants," u sun plants," 

 etc. 



585. Here may be noted the modifications of the 

 plant body following the acquisition of a parasitic habit. 

 These are well illustrated in the common Dodder (Cus- 

 cuta, a climbing vine related to the Morning Glories) 



