The Science of Plant Growing 



45 



seated. This is continued through the leaf, its stalk, the branches, stem, 

 and roots until a current of water, known as the "transpiration current", 

 is set up from the roots to the leaves. In the open air and on a windy 

 day this current is often so great that the roots cannot supply it, more 

 especially if the soil is dry, and the leaves flag as a consequence. This 

 phenomenon may often be observed in the case of pot plants if allowed 

 to get dry, whether under glass or outside. It can be remedied by water- 

 ing the soil in the pots and by syringing the foliage. The undue loss 

 of water from plants under glass can be more effectually prevented by 

 closing the ventilators before syringing, and shading may be resorted to 

 in extreme cases. The atmosphere then becomes saturated, thereby largely 

 checking transpiration for the time being, and the 

 leaves resume their wonted stiffness. This rapid ascent 

 of water serves to keep the plants cool in hot weather, 

 the cells turgid, and also brings in plant food. Trans- 

 piration is a vital process regulated by the protoplasm 

 in the leaves and is somewhat different from evapora- 

 tion pure and simple. For instance, a Stonecrop may 

 be placed between sheets of paper, covered by a board 

 and held down by a weight. It will continue to elon- 

 gate and even open its flowers under such conditions; 

 but if placed in a basin and some boiling water poured 

 over it to kill it the stems and leaves will part with 

 their moisture in a few days. At night the stomata 

 close, and transpiration ceases. Another phenomenon 

 may often be observed in the morning. Drops of 

 water may be seen on the tips of the leaves of Aspi- 

 distras, Arum Lilies, Fuchsias, Chinese Primulas, and 

 many others as a result of root pressure. Over the 

 ends of the vascular bundles of the leaves of those 

 plants water pores are situated, and unlike stomata they never close. The 

 roots continue to absorb water, night and day, and when transpiration 

 ceases the cell walls become saturated. Water then filters into the cavities 

 of the wood fibres and vessels under pressure from the roots until it 

 reaches the water pores, where it escapes from the overgorged tissues. 



Forms of Leaves and their Clothing 1 . Seed leaves are always simple 

 or in one piece, though they are lobed in a few cases. Simple leaves 

 (fig. 33) are represented by those of the Cherry, Apple, Fuchsia, and 

 Camellia. They may be more or less deeply and palmately lobed, as in 

 the Vine, Ivy, Sycamore, Plane, and Hop. The lobing may be in the 

 form of a feather, as in the common Polypody, Marguerite, Oak, and 

 Water-cress. This form is termed pinnatifid. The cutting is carried 

 still deeper in Celery, Parsnip, and Carrot, and the simple leaf termed 

 pinnatisect, or it may be twice pinnatisect in the Carrot. Leaves are 

 "compound" when each separate piece into which they are divided is 

 jointed, as in the Laburnum (fig. 34), the Virginia Creeper, Horse-chest- 



Fig. 33. -Simple Leaf 



p, Petiole, with Stipules 

 at the base ; 1, midrib ; 2, 3, 

 branches of the midrib. 



