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 bave 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,” “sun plants,” 
ete. 
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) 
