3] PHYSIOLOGICAL REACTIONS 93 



(a) Phatierophytes (tall aerial plants). Perennials, mostly trees or 

 shrubs, with their renewal buds on shoots at least 25 cm. (about 

 ID inches) above the surface of the ground, and hence exposed to 

 unfavourable weather. Phanerophytes are especially numerous in 

 moist areas of the tropics and subtropics, where they tend to pre- 

 dominate in the matter of numbers of species as well as individuals. 

 Elsewhere the species are usually few, even if the numbers of 

 individuals are great and their dominance is overwhelming. 



{b) Chamaephytes (surface plants). Perennial herbs and some 

 undershrubs with renewal buds between ground-level and a height 

 of 25 cm. — hence usually enjoying only such protection as may be 

 afforded by the plant itself or by snow, and consequently plentiful 

 in boreal and alpine regions. 



(r) Hemicryptophytes (half-earth plants). These have perennial 

 shoots and buds at ground-level or within the surface layer of soil, 

 etc., and hence protected by the habitat. Such plants are particularly 

 preponderant in high alpine and arctic regions but are also plentiful 

 in the temperate zone. 



{d) Geophytes (earth plants). These have the perennating organs 

 (such as bulbs, tubers, or rhizomes) well buried in the soil and there- 

 fore not exposed in unfavourable seasons. They tend to be com- 

 monest in temperate regions but also persist in fair numbers farther 

 north and south. 



{e) Hydrophytes (water plants). These include all water plants, 

 whether anchored or not, apart from microscopic free-floating or 

 swimming types which form the main basis of the separate category 

 known as ' plankton '. This group of hydrophytes tends to cut 

 across the other main ones and so is often omitted from ' spectra ' 

 {see pp. 94-5). 



(/) Therophytes (annuals). Plants which complete their life-cycle, 

 from germination to ripe seed, within a single limited vegetative 

 period, surviving the unfavourable times as seeds, spores, or other 

 special (usually resistant) reproductive bodies. They are especially 

 abundant in deserts where the unfavourable period may be par- 

 ticularly severe and prolonged, but are largely lacking in arctic 

 regions where the growing-season is too short or the warmth is 

 insufficient to allow them to complete development before winter 

 comes again. 



Examples of (b), (c), {d) and (/) are illustrated in Fig, 24. Almost 

 all trees and tall shrubs belong to {a), while examples of (e) were 

 illustrated in Fig. 21. 



