PHYSIOLOUY 



An unfailing test for starcii is a blue or blue- black 

 reaction on the addition of a weak solution of iodine. A 

 bit of starch ]i!istp. or the cut surface of a potato or 

 other stanliy uica, will i|uickly show this reaction. In 

 green Wn\ f^ >i:u< h may l)c tested by first dissolving out 

 the chlc.ii.|ili> II ill aN i.liol and then staining the leaf 

 with iodiiii-. Ill tin- saiiK.' way one examines a leaf varie- 

 gated with white. The green or colored parts of the 

 leaf alone will show starch, the white areas showing no 

 blue or purplish coloration, demonstrating that they 

 have formed no starch. 



It requires a glass apparatus, such as is shown in 

 Fig. 1784, to demonstrate that a land plant cannot form 

 starch in an atmosphere free of carbon dioxid. Over a 

 small potted plant (or better, the plant may be trans- 

 planted to a glass jar protected at the top liy rubber 

 3loth) is placed the vessel a, ci-iiHiit.-.l t.i tin- trlass 

 plate, 6. A solution of caustic poia^li in .■ ali-Mil,^ the 

 carbon dioxid in the vessel, and all air aiiiiiiiii.l must 

 pass through the U-tube d, which contains at < imiuice 

 stone soaked with caustic potash. This plant exposed 

 to direct sushine for a few days will show no starch 

 formation on testing its leaves. In a short time it will 

 also become unhealthy and ccasp ti. f;r..w. 



Not only does chlorophyll a. i in > ..njiiiMtion with sun- 

 light for the manufactur. ' ' ,t. in general, 

 sunlight is absolutely neiM-- ' liat .-hliirophyll 

 may be normally develii|M.i. >, ,,i, ^. i miiiiited in a 

 darkened vessel or potato .-vi-iuui.^ uh.rh liuve pushed 

 into growth in a darkened cellar will remain yellow or 

 white. Moreover, the plants will grow long and slender, 

 and death will result when the plantlet can no longer 

 draw upon the parent part for starchy matters. The 

 total dry weight of such jdaiits will m.t be greater than 



the dry weight of the orinmal - I .ir tulnr. In this 



connection it might be sTatr.l that t'l rn spores require 

 some light in order that f;.i niiiiaiiun may occur, while 

 the germination of <iriliiiar\ Il.iwfring jilants is slightly 

 retarded in the pn-siia <■ nf li-lit. 



The sugar into whicii -lai.li is la.nverted for translo- 

 cation is abundant in tin- I. av.s; and it is also trans- 

 ferred to all liviii- parts ni tlir |,lant. alniiffwith other 

 org_anic pr.i.lnrts, l„si,|,.s tin- varii.iis salt- in -uhition 

 which hav.' , ,.i,,i- n]< u, ih.' Icav,- tnnn tin- s,,il. ruder 

 the inHu.n t llir anivr |,i-,.tM|ilasni .,]' til.- l.aif-.'clls 



in growth may be formed. All parts of the ordinary 

 plant are dependent upon the roots for a supply of the 

 mineral salts and nitrogen ; but, on the other hand, 

 they are entirely dependent upon the leaves for the first 



organic substance, and for much iiiii.arfd f 1. 



Growth, and the Differentiatin,, ,,i .s7,», /„,-,.- Plant 

 growth is apparent to the unaided .y,- as ,liaiiL'<- in 

 form and size of organs and tissuis. Tin- r.-al tvi- 



dence of growth Is in the multiplication of the tissue 

 cells, or of constructive changes in the form and bulk 

 of these cells. Growth may be so rapid that it may be 

 readily measured, or it may proceed so slowly or by such 

 obscure internal modifications that very little external 

 Indication of the complex processes will be manifest. 



PHYSIOLOGY 1325 



Even under such adverse conditions as that of decidu- 

 ous trees in winter, some slight growth may be taking 

 place, and it is not well to dissociate from the idea of a 

 living plant all growth phenomena. Nevertheless seeds 

 and other air-dried plant parts may live without growth 

 for considerable periods. 



The growth in size and length of different plant or- 

 gans is very various. The zone of growth in the root is 



it back of the tip, so that if an ink mark 



made immediately behind the tip and 



another a quarter of an inch further back, 



almost the entire growth extension of the 



will take place within the region thus 



very tip, because 

 1 to aid in forcing 

 id obstacles. The 



marked. There is no ^M-u\\tl] 

 it is hard and protect. .1 liy a 

 its way through the sni! aiai ai 

 region of greatest cell divisi.i 

 region of greatest elongation. ( in tlir otli.r Jiand. the 

 young stems of annual plants and tin- rapiil .■l.iiii;aiion 

 of the young shoot may for a tiiia- s1m>\v lh-ow ili ilircn^^h- 

 out the entire extent. The wini.r cuialiti.in ,,t' a l.< . ,h 

 twig is shewn in Pig. 1785, and the leiig, .klicate, 

 <.v.rla|.|.in- sralcs of the buds are very evident. Each 

 hiiii iv an iiiri|.iiiit branch, as is readily seen in the 

 spring' will 11 ihi- liiids elongate; the delicate scales 

 si-jiarair t'arilnr and farther from one another, each 

 bearinir a litilr haf in its axil, and marking a now 

 distinct joint .ir iii.ili- in the new branch. Various 

 stages of this L'.ii. ral ilMngation are evident from 

 Fig. 1786. Finally as tin- branch lengthens through- 

 out its entire extent, the scales drop, the leaves expand, 

 the older nodes cease to elongate, and the wave of elon- 

 gation follows a few nodes behind the terminal bud. 

 In the common woody plants growth in diameter is 



of 



ter of the 



forms, or else peels off periodically. 'I'li. I it. n nn ,1 i.n 



of the tissues in different parts denote- 1- 



ological functions. Thus the woody ]mii ■ ■ ■ hl' 



rings conducts the water and other -> il ! i-. aiid 



through the woody bundles of the leaf-stalk, veins, and 

 veinlets it is distributed throughout the plant body. 

 The bark or phloem portion of the bundles is largely 



