PHYSIOLOGY 



the 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 



PHYSOCARPUS 



1327 



cliill 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 functions, and from the leaf are gradually withdrawn 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. 1ti tliis |in«-. ss tlic chlorophyll is oxidized 

 and changes frcMii -i.n i<. -"iiic other color, as yellow 

 or red; and oft. n it is l,\ ilii, means that the beautiful 

 autumn colors ar.> .1. v. i(,|n.l. These colors also serve 

 very likely as a proti-ctmn 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 

 bv a detrr.i- of .■old less than the freezing point. It is 

 becau-r III- iii"i.rl:i-iii "f til- '■■■IK i- stimuliiti-d to 



give HI- ■ Hi,- s|,:,r,.s l„.tw.-. 11 til. .ells, and 



then IP' ■ ■ ■ ■ I'. 111.' |inii..|.|i.-iii. ihis water is 



rcadiU . ,,..i.,:..j I ilie |.l,iiit .li.^ fimn li.in;; dried 



out. in li.i .-aiui ::.,;. a plant may wilt ami eventually 

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

 general, freezing consists in the dr>-ing 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 llie .lifferin;,' eliiirai'ter of the 

 vegetation indifferent III', -m . T. m i" mi m . is ii,,i 



alone the cause of the .1 111' ■ ' i|i;.l 



factor. In the tropics ,. , . i i ,■ : ; ■ . :. nniiiii.'. 



and gigantic leaf surfa. . • ,,,. .- ,, i iiiii..n 



to the great moisture cuum ..t an ami ...il. la 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 dift'ering capacities of the 

 plants to thrive in differing depths of water, or degrees 

 of moisture, cause them to be arranged in definite 

 colonies or zones. 



The sum of the responses to these and other stimuli 

 determine the form and character of the plant, and 

 determine whether it shall very closely resemble its an- 

 cestors, or whether it shall have characteristics vary- 

 ing slightly from them. From the same parent a dande- 

 lion of the mountain-side will differ somewhat from the 

 dandelion of the lowland meadow. External agents, 

 UTider which category cultivation is an important factor 

 with .I.imesticated plants, act not only slightly to change 

 iiiiliviiluals,but in time to change varieties and species. 

 W.irkiiif,' from one generation to another, in conjunction 

 with natural or artificial selection, external agencies 

 develop new forms and habits as the plant adapts itself 

 more perfectly 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 Linnaean times. 



Literature.— AmoDg works upon plant physiology may 

 he 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 

 "Physiology of Plants," translated by Ewart (Clarendon 

 Press). The first mentioned is intended for the use of 

 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," by Lind- 

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

 similar works may be consulted with much profit. 



B. M. DOGOAB. 



PHYSOCAEPUS (Greek, physa, bladder, and karpos, 

 fruit; alluding to the inflated capsules). Syn., Opxi I as- 

 ter. Sosaeew, tribe Spirceetc. Ninebakk. 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 shrub])eries 



1790. Ninebark 



and grow in almost any soil. Prop, easily by either 

 hardwood or greenwood cuttings, also by seeds. Five 

 species in N. America and Amurland, allied to Spira-a 

 and formerly mostly referred to this genus, but distin- 



