GROWTH AND BRANCHING. 



45 



upwards especially through the young woody 

 tissue, thus mounting in opposition to the force 

 of gravity. As soon as the leaves expand the 

 superfluous water is removed by transpiration, 

 and thus the upward current, with its fresh 

 supplies of mineral matter, is promoted. The 

 cause of the ascent previous to the unfolding of 

 the leaves is still imperfectly known. Arrived 

 in the leaves, and exposed to the light, the 

 water, together with the mineral matters in 

 solution, take part in the complex chemical 

 processes that go on in the leaf, the superfluous 

 water, as before stated, being exhaled. 



The substances formed in the leaf pass into 

 the branches, reach the buds or other parts 

 where growth is going on most actively, e.g. in 

 the cambium tissues, or, if not wanted for im- 

 mediate use, are stored up in the bark or in 

 tubers and other receptacles. The phloem or 

 bast tissues and those of the inner bark are 

 those most actively concerned in the passage of 

 these nutrient fluids. Hence, if a tree be ringed, 

 that is, deprived of its bark so that the wood is 

 exposed, the communications will be interrupted 

 and the tree will die; but if the arrangement 

 of its phloem elements be such that they are 

 dispersed in the old wood, then the girdling or 

 ringing is not injurious, as communications can 

 still be maintained. Palm-trees and other mono- 

 cotyledonous plants are not so much injured as 

 others by the constriction of climbing plants, 

 for the same reason. 



The Sap. — The word "sap" was at one time in 

 general use by botanists, as it still is by gardeners, 

 and a distinction was further drawn between the 

 ascending and the descending sap. But as the 

 words in question lead to the false inferences 

 that there is a fluid of uniform constitution 

 ascending in a regular current, and a similar 

 fluid of equally definite nature descending in a 

 similarly regular channel, it is as well to abandon 

 the use of such misleading expressions. There 

 is no true circulation in plants, in the sense that 

 there is in animals or in a hot-water apparatus, 

 although there is in spring undoubtedly a marked 

 upward current. There is no one fluid (like the 

 blood of an animal) of uniform composition 

 throughout the plant, the juices at one part 

 being of a different constitution from those else- 

 where, and varying also according to season 

 and age. 



There is also a passage of fluids in a down- 

 ward direction, but not definitely or exclusively 

 so throughout the plant. The juices, in fact, 

 are transferred from point to point according to 

 different requirements, irrespective of direction. 



The passage of fluids in different directions is 

 facilitated by the movements caused by the 

 wind. Plants are rooted in the soil, and have 

 not the opportunity of taking exercise that 

 animals endowed with the power of locomotion 

 have, but this is partly compensated for by the 

 swaying to and fro of the young shoots. 



A certain amount of heat is necessary to excite 

 the flow of the juices, as before mentioned, and 

 this is not confined to the root, for the shoots 

 of a vine, for instance, may begin to grow and 

 to push forth leaves under glass when the roots 

 are in an outside border. [m. t. m.] 



CHAPTER VII. 



GROWTH AND BRANCHING. 



Necessary Conditions — Definite and Indefinite 

 Growth — Buds — Sympodes — Cuttings — Grafting 

 and Budding — The Inflorescence and Bracts. 



As a consequence of the co-operation of 

 favourable conditions of temperature, moisture, 

 and light in soil and air, and as a result of the 

 processes before described, growth ensues. This 

 growth may be a mere extension or increase of 

 the protoplasm and cell-walls, or it may be the 

 result of the rapid formation of new cells by 

 subdivision of the old ones. In either way the 

 plant may extend itself, when not obstructed, in 

 any direction. If the force of growth be equal 

 in every direction, a spherical form results; 

 if growth be chiefly in the horizontal plane, a 

 flat plate or thallus is produced. If growth in 

 length predominate over growth in breadth, 

 then we get long thread-like forms. When 

 growth is more active at the circumference 

 than in the centre, a hollow tubular or cup-like 

 form is assumed. If growth be more energetic 

 on the upper of two surfaces, then the upper 

 surface is convex, ^ ; or, on the contrary, if the 

 growth-force is greater beneath, then the lower 

 surface is convex, ^. Of course the intensity 

 and direction of growth vary according to times 

 and circumstances, and thus are produced the 

 various curvatures that we meet with in plants. 



When the structure is uniform, the two sur- 

 faces are alike; but when there is a difference, 

 the organ is said to be dorsi-reutral, the dorsal 

 surface being that which is directed away from 

 the apex or growing point, the ventral surface 

 being that which is directed towards the centre. 

 The dorsal surface is generally also the lower or 

 the outer surface, while the ventral is the upper 

 or inner face. 



