264 BOTANY rAiix i 



at the expense of the stored water and can often withdraw the 

 water from older portions so that these wither while growth goes on 

 at the apex. 



Plants in damp situations are usually larger than those grown in dry places, 

 and in fact may differ from them in their whole habit and mode of growth. Direct 

 contact with water seems frequently to exert a special influence upon the external 

 form of plants. Amphibious plants, that is such as are ca])able of living both 

 upon land and in water, often assume in water an entirely different form from that 

 which they possess in air. This variation of form is particularly manifested in 

 the leaves, which, so long as they grow in water, are frequently linear and sessile 

 or finely dissected, while in the air their leaf-blades are much broader and provided 

 with ])etioles (cf. Fig. 37). The leaf-stalks and internodes also often exhibit a 

 very different form in air and water, and undergo the same abnormal elongation as 

 in darkness. This is especially noticeable in submerged water plants, whose organs 

 must be brought to the surface of the water (young stems and leaf-stalk of Trapa 

 natans, stem of Hippiiris, leaf-stalk of JVymjihaea, Nuphar, Hydrocharis). Such 

 plants are enabled by this power of elongating their stems or leaf-stalks to adapt 

 themselves to the depth of the water, remaining short in shallow water and becom- 

 ing very long in deep water {^''). 



The great importance of free oxygen has already been alluded to (p. 211). 

 Without gaseous or dissolved oxygen in its immediate environment the growth of 

 a plant entirely ceases, at least in the case of aerobionts. Variations in the pro- 

 portion of oxygen in the air also influence growth. 



Among CHEMICAL STIMULI the poisons {^^) must first be mentioned ; these are 

 substances which in very dilute solutions arrest growth and idtimately life. It is 

 a striking fact that many of them when in extreme dilution have a stimulating 

 effect on growth. Chemical stimuli due to other substances play a large part in 

 the germination of many seeds, spores, and pollen grains, and in the develo})meut 

 of fruits. Some pollen grains only germinate when they obtain traces of substances 

 which are present on the stigma. Many parasitic fungi and also parasitic phanero- 

 gams {Orohanche, Lathrea) are stimulated to develop by unknown substances pro- 

 ceeding from tlieir hosts. In Algae and Fungi high concentration of some food- 

 materials may give rise to striking changes in form C^"). 



The many influences which occur in symbiotic or parasitic association of organisms 

 are probably, when analysed, of a chemical nature. Hypertrophies or new formations 

 (galls) result from them (cf. pp. 170, 252). 



Growth may also be strongly modified by corkelations. It can only be 

 mentioned that the unfolding of one axillary bud in a Phanerogam may hinder the 

 growth of many others ; if, however, the dominant shoot is removed the arrested 

 buds proceed to grow farther. Other internal conditions in which correlations 

 certainly ]ilay a jiart may modify the ibrm of the plant. For exani})le, in shoots, 

 an elongation of tlie iuternodes and a reduction of leaves, such as is seen in etiolated 

 plants, may result from the development of flowers or the transition to a twining 

 habit of growth. We shall scarcely go wrong in considering the determining 

 influences in these cases also as of the nature of chemical stimuli. 



Mechanical Influences. — Pressure and traction exert a purely mechanical 

 influence upon growth, and also act as stimuli uiion it. External pressure at first 

 retards growth ; it then, however, according to Peeefeu, stimulates the proto- 

 plasm and occasions the distension of the elastic ceTl walls, and frequently also 

 an increase of turgor. As a consequence of tliis increased turgor, the counter- 



