PHYSIOLOGY 



tne leaves on that side of the tree from which the wind 

 comes are much more injured than others. 



The annual fall of the leaf in deciduous trees is usu- 

 ally a matter of temperature, although drought and 

 other conditions may also cause periodic defoliation. 

 It would cost much labor to protect the large green sur- 

 faces during the winter and it is economy to part with 

 a portion of the structure. The cool days of autumn 



PHY^OCARPUS 



1327 



maple injured by 



chill the root-hairs and irritate the assimilatory organs. 

 The former cease to perform their normal absorptive 

 functions, and from the leaf are gradually withdrawn 

 the substances which are readily made soluble. The 

 cell walls and the less useful parts are left, and by the 

 formation of a distinct corky layer across the leaf-stalk, 

 where it is attached to the main stem, the plant cuts off 

 its assimilatory organs by a natural process, so that no 

 wound or injury except the well-healed leaf-scar shall 

 mark its fall. In this process the chlorophyll is oxidized 

 and changes from green to some other color, as yellow 

 or red; and often it is by this means that the beautiful 

 autumn colors are developed. These colors also serve 

 very likely as a protection to shield twigs and trunks 

 from the hot autumn sunshine. Likewise, the twigs 

 themselves may be provided with color for the same 

 purpose. 



The old leaves are dropped in the autumn, but al- 

 ready a new set of modified leaves in the form of bud- 

 scales have been formed, in turn to be defoliated the 

 next spring, after serving a term of winter protection. 

 Deciduous trees then shed their summer leaves when 

 growth ceases and their winter leaves when growth is 

 awakened. 



Plants such as the squash and potato may be killed 

 by a degree of cold less than the freezing point. It is 

 because the protoplasm of the cells is stimulated to 

 give up its water into the spaces between the cells, and 

 then not being held by the protoplasm, this water is 

 readily evaporated and the plant dies from being dried 

 out. In the same way a plant may wilt and eventiially 

 be much injured if cold water is applied to its roots. In 

 general, freezing consists in the drying out of the pro- 

 toplasm and the formation of ice crystals between the 

 cells. The plant may recover if the protoplasm can 

 gradually reabsorb this water: it will die if the water is 

 not reabsorbed. 



The effect of temperature upon orders and species of 

 plants is very evident in the differing character of the 

 vegetation in different life zones. Temperature is not 

 alone the cause of the difference, but it is the principal 

 factor. In the tropics succulent plants predominate, 

 and gigantic leaf surfaces abound as accommodation 

 to the great moisture content of air and soil. In the 

 temperate regions there is a degree of heat encouraging 

 perfection of size in woody development coupled with a 

 considerable luxuriance of foliage, as well as a large 

 development of herbaceous plants. In arctic regions 

 the more succulent green growth is entirely suppressed, 

 in general smaller woody forms abound, and even the 

 texture of leaves and fruit is expressive of hardiness. 



Plants along the seacoast differ from those farther 

 inland, the salt spray having a very injurious effect 

 upon those which have not become resistant to it. 



Around the edges of ponds and lakes there is a struggle 

 for position, and as a rule the differing capacities of the 

 plants to thrive in differing depths of water, or degrees 

 of moisture, cause them to be arranged in definite 



The sum of the responses to these and other stimuli 

 determine the form and character of the plant, and 

 determine whetlier it shall very closely resemble its an- 

 cestors, or whether it shall have characteristics vary- 

 ing slightly from them. From tbf^ same parent a dande- 

 lion of the mountain-side will ditV. r s,„,i,.« i,;it from the 

 dandelion of the lowland m.ail"\v. i;\i. rnul agents, 

 under which category cultivatiun i- an iiniM.iiant factor 

 with domesticated plants, act not only sli^'lnly to change 

 individuals, but in time to change varieties and species. 

 Wiirkiiiir from one generation to another, in conjunction 

 with natural or artificial selection, external agencies 

 ilevelnp new forms and habits as the plant adapts itself 

 more jK-rfectly to these conditions. In this way plants 

 vary as individuals, and in time as races or "species. 

 These variations are but slight from one generation to 

 another, but it is safe to say that there are few culti- 

 vated plants to-day which resemble exactly their ances- 

 tors of the Linnfean times. 



iiVfi-n^Hj-e.— Among works upon plant physiology may 

 be mentioned Sorauer's "A Treatise on the Physiology 

 of Plants," translated by Weiss (Longmans, Green & 

 Co. ) ; Detmer's "Practical Plant Physiology," translated 

 by Moor (The Macmillan Company); and Pfeffer's 



gardeners, and the others are technical treatises. Such 

 books as "Living Plants and Their Properties." by 

 Arthur & MacDougal; "The Survival of the Unlike." 

 by L. H. Bailey: "A Theory of Horticulture," bv Lind- 

 ley; "Plant Kelations," by J. M. Coulter; and other 

 similar works may be consulted with much profit. 



B. M. DUGGAR. 

 PHYSOCAEPUS (Greek, physa, bladder, and kai-pos, 

 fruit; alluding to the inflated capsules), ^jn., Opithis- 

 ter. Bosacecp, tribe Spirwea. Nixebark. Hardy orna- 

 mental deciduous shrubs, of spreading or erect habit, 

 with stipulate, alternate, petiolate and mostly 3-lobed 

 Ivs. and with umbel-like heads of whitish fls. appearing 

 late in spring, terminal on short branchlets along the 

 stems and followed by clusters of small pods, inflated 

 in some species and often assuming a bright red color 

 late in summer. Thev are well adaoted for shrubberies 



1790. Nmcbark— Physocarpus opulifohus (X K) 



and grow in almost any soil Prop easily by either 

 hardwood or greenwood cuttmgs also by seeds. Five 

 species in N. America and Amurland, allied to Spiraea 

 and formerly mostly referred to this genus, but distin- 



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