466 



HORTICULTURE 



November 3, 1906 



Plant Tissues 



PART 



lu previous articles I have classified plant tissues, 

 discussed the three regions of gi'owth in one year old 

 twigs and described the different tissues of which each 

 season's twigs are composed. In the present article I 

 shall trace the development of these tissues and corre- 

 late form with function and in a final article discuss 

 secondary thickening or increase in- diameter from year 

 to year, explain bark development and cork formation 

 and conclude with some recently published records of 

 bud growth in fruit trees. 



As stated before tlie terminal cap of formative tissue 

 at the growing point produces all the various stem ele- 

 ments of the season's twig except those resulting from 

 the later growth of the cambium tissue in the conduct- 

 ing bundles. The elements of the twig besides the 

 bundles are fundamental tissue and epidermis. The 

 fundamental tissue includes the pith, pithrays, cortex 

 and strengthening tissue. The areas of strengthening 

 tissue such as the bast fibres formed near the bundle or 

 the other long thickwallod cells developed in the cortex 

 or elsewhere in the stem arc simply fundamental tissue 

 later modified as internal protective tissue. 



The pith, pitliravs and cortex are all quite large cells 

 well filled with protoplasm and provided with thin walls 

 having in some cases many shallow pits. They become 

 larger during their ])eriod of elongation just back of the 

 growing point but later undergo very little differentia- 

 tion. Their function is to conduct and store food, 

 mainly starch. This, in the form of sugar solution, can 

 easily pass through the thin places at the piti and 

 therefore there is no need for such open perforations as 

 are found in the sieve tubes of the phloem. The starch 

 passes down from its plac(> of manufacture in the leaves 

 through the cortex and phloem and is carried inward 

 radially through the pith rays which are elongated in 

 this particular direction. The pith rays convey the 

 starch not only to tlie pith cells but also to the living 

 wood cells of the bundles. The function of these thin 

 walled wood cells is not very well understood at present. 

 In the winter twigs of many trees all these thin walled 

 tissues show the Ijhie iodine test for starch stored here 

 as reserve food lor early spring use. 



The epidermis undergoes much more differentiation 

 tliian the fundamental tissue. At first they are thin 

 walled but later become modified to function as external 

 protecting tissue and as water storing tissue. The 

 water is held in by the thin lining membrane of proto- 

 plasm and their outerwalls become gi-eatly thickened 

 and cutinized. The chief service of this thickening is 

 to prevent excessive transpiration of water and restrict 

 its passage to the breathing pores or stomates. These 

 openings occur in the c[>idermis of stem and leaf and 

 are guarded l)y elaljorate cells wliich are sensitive to the 

 loss of water. The combined action of the guard cells 

 opens or closes the stomates and thus regulates the 

 evaporation of water from the plant. 



The stomates also allow the interchange of gases 



III. 



necessary for plant respiration fur plants breathe 

 as truly as animals do. The stomates are the breathing 

 pores through which air is admitted to the working tis- 

 sues within. During day time when starch formation 

 is going on in the green parts of the plant the impure 

 carbon dioxide breathed out within the working tissues 

 is at once used up in the manufacture of starch and 

 the additional carbon needed is extracted from the car- 

 Ijon dioxide of the air taken in through the stomates. 

 When such work is going on rapidly there is actually 

 a surplus of oxygen thrown off by the plant. In the 

 day time plants really tend to purify the atmosphere in 

 which they grow. At night when starch is not being 

 formed metabolism, transportation, growth and other 

 vital processes are still going on and the working tissues 

 are taking in oxygen and discarding carbon dioxide. 

 The exhaled carbon dioxide is now a surplus gas within 

 the plant and is passed out into the surrounding air. 

 At night plants tend to make the air impure just as 

 animals do all the time. 



The development of the fibrovascular bundles begins 

 at the growing point from certain radiating parts of the 

 formative cap called procambium strands, the cells of 

 which are much smaller than those of the surrounding 

 tissue. Some of the procambium cells early become 

 differentiated into the first xylem elements on one side 

 of the procambium strand and the first phloem elements 

 on the other side. The remaining undifferentiated cells 

 between continue to divide longitudinally and consti- 

 tute the cambium. When one of the procambium cells 

 passes over into cambium tissue it divides longitudinally 

 and one part remains nascent, i. e., stays a cambium cell 

 while the other ])art becomes a mother cell of xylem or 

 phloem and may divide and subdivide several times pro- 

 ducing a number of cells. These cells now become dif- 

 ferentiated into )iermanent xylem or phloem elements 

 of the bundle without further division. During the rest 

 of the growing season the cambium continues dividing 

 and forms the later liundle elements, xylem or wood on 

 one side and piiloem or bast (sometimes called inner 

 bark) on the other side. 



The first phloem elements formed by the procambium 

 at the growing point arc not different from those formed 

 later by the cambium tissue. The function of the sieve 

 tulies of the phlncm is to conduct proteids, i. e., albumi- 

 noid or nitrogenous solutions. These solutions can not 

 readily l)ass through even the thin membranes at the 

 ])its and therefore tlie sieve tubes of the phloem are 

 provided with open perforations in their side and end 

 walls. 



In the xylem elements, however, there is great differ- 

 ence between those formed first by the procambium and 

 hiter by the cambium. The first formed elements are 

 the large, spiral or annular vessels with a few of the 

 long, thin walled tracheids. The function of these first 

 wood elements is (o conduct the watery soil solutions 

 \ipward through the stem. Their walls must be tlun 

 so as to allow of rapid elongation back of the growing 

 ])oint and yet strong enough to keep the water channels 

 always open. At first the early formed spiral or annu- 

 lar vessels arc uniformly thick walled but when elonga- 

 tion takes place the wall breaks along a weak spiral or 

 annular line, ail except the central layer called the mid- 

 dle lamella. This at first stretches out to aecotnmodate 

 the elongation but may eventually break also. There 

 is thus produced a thin walled vessel with spiral or an 

 annular thickenings which still serve to keep the 6han- 



