and implies starvation. Addition of Nitrates stimulates proteid-synthesis, and so reduces 

 polysaccharide deposits, giving rank but often mechanically weak growth. Phosphorus 

 stimulates nuclear activities. A certain relation between Ca, K, Mg suggests that these 

 may be individually toxic, but that they neutralize each other, as possibly inherited 

 from sea-water. 



Water Problems are obviously insistent for all subaerial vegetation, but the 

 water-problem is but a part of the wider problem of the source of food-salts and 

 combined nitrogen. Special cases attract attention, as evaporation becomes more 

 acute. Although all land-plants require to transpire excessively to obtain food-salts, 

 regulation is required. Plants in danger of losing water faster than they can replace 

 it are termed Xerophytes ; and commonly present striking Xeromorphic adaptations 

 in form, anatomy, and general habit. 



The leaves as the essential transpiring organs are the first to be affected ; followed 

 by the entire shoot-system. As available examples, cf. : (i) Leaves reduced in area 

 (Box, Heath); (2) Reduction of intercellular spaces (Conifers); (3) Diminution in 

 immber of stomata (Aloe, 10-20 per sq. mm.); (4) Thick cuticle (Holly, Yucca); 

 (5) Deposits of wax ('bloom' of Cabbage); (6) Clothing of hairs ( Verbasctim) ; 

 (7) Rosette-habit, with overlapping leaves (Sempervivuni) ; (8) Leaves edge to light 

 (Eucalyptus), or vertically orientated (Iris] ; (9) Extreme succulence by osmotic effect 

 (Sedum} ; (10) Sclerosis of covering-sheets of lignified tissue (Pinus) ; (i i) Entire loss 

 of leaves (Cereus); (12) Replacement of leaves by flattened cladodes (ppuntia\ 

 phyllodes (Acacia sp.), or phylloclades (Ruscus, Asparagus}; etc. 



Also any combination of such factors, producing characteristic xerophytic vegeta- 

 tion, where such conditions are predominant throughout the year. But a plant may 

 be xerophytic (i) at one period of ils life and not another; (2) at some time of the 

 day; or (3) at some particular season: examples as (i) Seedlings with feeble root- 

 system; (2) Herbaceous plants wilting in hot afternoon; (3) The evergreen tree 

 (Holly) with feeble root-absorption in winier, and the deciduous tree, as the common 

 example of the N. Temp, region, shedding leaves when the transpiration-system fails. 



Special cases of Aquatics and Hygrophytes present variants on the water- 

 problem, due to insufficiency in the supply. Aquatics, with all subaqueous parts 

 rooted in mud, impoverished for free oxygen-supply (cf. Hippuris\ utilize a voluminous 

 internal atmosphere in lacunar spaces, with free communication to subaerial portions 

 of the body. 



Transpiration is wholly eliminated in plants growing in a saturated atmosphere, 

 or entirely submerged : in such case (cf. Zostera under the sea), salts can be obtained 

 with difficulty by direct adsorption; or by active secretion of excess water, as in 

 Nepenthes of tropical rain-forest. 



Orientation of the main stem is effected largely in terms of positive Helio- 

 tropism ; but also in terms of response to stimulus of gravity, as negative Geotropism. 

 The plasma of stationary land-vegetation becomes sensitive to the direction of the fall 

 of its own material particles; and the response to this sense of direction may be 

 positive, negative, or transverse (plagiotropic) ; or even vary from time to time in the 

 same organ. Interpretations of geotropism in terms of ' falling starch', popular since 

 1 900, as hypotheses of ' statocyte ' nature, remain purely speculative. The cells of 

 a growing apex, with maximum sensitivity, have neither extensive vacuoles nor starch- 

 grains. The mechanism of response, by a growth-curvature, can be only effective in 

 regions in which the cells present a capacity for dilatation and extension. 



Note, that changes in environment can never be postulated as Causal : the 

 individual plant is not an isolated mechanism, but the result of ages of inherited 

 response to similar stimuli, fixed by natural selection as part of the present 

 equipment of the race. Just as it is quite unjustifiable to read into the plant 

 ideas of thought, prevision, and design, based on our own more elaborated percep- 

 tions and actions ; so it is equally wrong to regard the plant as a mere physical 

 mechanism, and the passive victim of environment. The object of physiology is to 

 hold the balance between these two standpoints, with a view to understanding what is 

 the nature of living response. An inherited capacity for adaptability, within a certain 

 range, with every change of condition, may be termed the Vitality of the organism, 

 ultimately translated as self-determination. The mechanism of such inheritance, and 

 racial progression, is included under the heading of Keproduction. 



24 



