294 BOTANY PART I 



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 (stem of Hippuris, leaf- stalk of 

 Nymphaea). 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 becoming very long in deep water. 



The water-forms also differ from the land-forms in their internal structure. 

 Thickened cell walls are frequently absent from the stem, and the vascular bundles 

 are reduced; the leaves resemble shade -leaves. The most marked contrast to 

 water plants is presented by such land plants as are exposed to insufficient water 

 supply or too active transpiration. In these the vascular bundles are strongly 

 developed, while the epidermis has the arrangements which have been considered 

 under the means of protection against excessive transpiration. 



In addition to the true nutrient materials which are employed 

 in the construction of the substance of the plant, oxygen requires to 

 be mentioned. Although its entry into the plant is connected with a 

 loss of organic substance, it is quite indispensable for growth on 

 account of the need of respiration. In aerobic plants at least, growth 

 ceases completely on the withdrawal of oxygen ; a diminution or 

 increase of the proportion of oxygen in the air also influences growth. 



Stimuli of the most various kinds proceed from substances acting 

 on the plant. 



Poisons must first be mentioned ; these are substances which in very dilute 

 solutions arrest growth and ultimately life. Thus even in a dilution of 1 in 

 25,000,000 copper sulphate kills such Algae as Spirogyra and also peas in water 

 cultures. It is a striking fact that many poisons 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 

 development 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 Phanerogams (Orobanche, Lathrea) are stimulated to develop by unknown 

 substances proceeding from their hosts. In Algae and Fungi high concentration 

 of some food materials may give rise to striking changes in form. 



6. Influence of Foreign Organisms. Fungi and Bacteria living 

 parasitically in flowering plants often cause profound deformations 

 that are known as GALLS ( 60 ). In the simplest cases there is merely a 

 hypertrophy of cells, while in more complex ones there are qualitative 

 changes in the organ. Still more striking gall-formations are caused 

 by animals, especially insects. Outgrowths form, which serve the 

 parasites for protection and food. The structure of the gall appears 

 purposive when considered from the side of the parasite, the protective 

 layers and nutritive layers of the gall being without significance for 

 the plant. 



Euphorbia Cyparissias, when attacked by a rust fungus (Aecidium Euphorbiae), 

 becomes sterile, remains unbranched, has shorter and broader leaves, and in its 

 whole appearance is so changed as scarcely to be recognisable. Plant lice some- 

 times cause a flower to turn green, so that instead of floral leaves green foliage-like 



