October 13, 1906 



H ORTI CULT 



Ti 



391 



Plant Tissues 



PART II. 



lu our last article wv gave a brief classification of 

 plant tissues. We will now speak in detail of formative 

 plant tissues and discus.s how a tree grows in height and 

 diameter. 



The end of a growing shoot may be divided into three 

 regions. First, the extreme tip occupied by a leaf or 

 flower bud in which new parts are being developed. This 

 is the formative region. Second, the elongating region 

 just back of the formative region. And third, the ma- 

 turing region, a region of differentiation in which the 

 tissues cease to grow in size and begin to assume the 

 peculiar form and texture necessary for the kind of 

 work which they are to do. 



Plants protect their tender growing points from win- 

 ter injury by the formation of winter buds. A winter 

 bud is a covered, resting growing point. When spring 

 comes the bud opens and the protective bud scales are 

 shed leaving a scar which marks the end of the last 

 season's growth. Sometimes these bud scars are visible 

 for several years back. The twig continues growing for- 

 ward at the tip, each year adding much if the season 

 is favorable or little if conditions are adverse. But the 

 twigs of last year never become any longer nor do the 

 limbs of a tree grow further apart or higher from the 

 ground. All growth in length is limited to the region 

 of elongation just back of the growing point. 



The terminal cap of formative tissue at the growing 

 point produces all the various stem elements of the sea- 

 son's twig except those resulting from the later growth 

 of the fibrovascular bundles. A cross section of such a 

 twig made near the close of the growing season will 

 show the following structures. First, the continuous 

 ring of epidermal cells on the outside. Long before 

 this the outer walls of the epidermis have become 

 greatly thickened and heavily cutinized, i. e., in- 

 filtrated with cutin, a waxy product, thus restrict- 

 ing the passage of moisture to the regular channels, 

 the breathing pores or stomates. A superabundance of 

 cutin produces the bloom on grapes, the glaucous appear- 

 ance of plant stems and the wax gathered from the ber- 

 ries of Myrica cerifera. Second, a smaller concentric 

 ring of conductive or fibrovascular bundles and, thirdly, 

 large, thin walled, fundamental tissue in whicli the con- 

 ducting bundles are embedded forming the pith in the 

 centre of the twig, the cortex which separates the bun- 

 dles from the epidermis and the pith rays which run 

 between the bimdles and connect the cortex with the 

 pith. 



Eacli fibrovascular bundle is composed of xylem, 

 phloem, cambium and strengthening tissue. The xylem 

 is the woody part of the bundle and lies next the pith. 

 The phloem is the bast part of the bundle and lies next 

 the cortex. The cambium is the active formative part 

 of the bundle and lies between the xylem and the 

 phloem. The strengthening tissue has developed in two 

 areas, one outside the xylem next the pith and the other 



outside the phioeui next the cortex. The strengthening 

 tissue is characterized by long cells with very thick 

 walls. 



The xylem is composed of large ducts or vessels, long 

 pointed tracheids and short blunt wood cells. The ducts 

 are composed of large thin walled cells placed end to 

 end from wliich the cross walls have early disappeared. 

 Thus open channels are formed which extend for long 

 distances through tlu' stepi. Their side walls have 

 spiral or annular thickenings to keep the cells from 

 collapsing and are also provided with numerous circular 

 thin places called pits to facilitate the passage of 

 liquids. The tracheids are cells not ducts. They are 

 smaller than the ducts or vessels and have sloping cross 

 walls which give them larger and stronger end contact. 

 Those formed near the ducts are thin walled with sim- 

 ilar thickenings and pits in their sides. The wood cells 

 are small ^vith square cross walls and many pits. 



The cambium cells are xevj small with thin walls but 

 having ceased active growth are seen to better advantage 

 earlier in the season. The phloem is marked by the 

 presence of sieve tubes which are long, thin walled 

 cells with perforations in their side and end walls ; asso- 

 ciated with the sieve tubes are smaller companion cells 

 whose function is not loiown. Having seen what the vari- 

 ous elements of a one-year-old twig are like we are now 

 ready to trace their development and to correlate form 

 with function. 



Park Dept., Boston, Mass. 



(Concluded in issue of October 27.) 



Clematis heradeifolia stans 



I had the pleasure this summer of examining speci- 

 mens of the Clematis heradeifolia stans grown by Mr. 

 G. F. Dwyer of Lynn, and illustrated in your issue of 

 September 29. I compared these specimens carefully 

 with our clematis and find them considerably alike. Our 

 clematis referred to is the one described last year in 

 HoRTicuLTUiiE Under the name of Clematis davidiana. 

 In that brief article it was intimated that this name 

 was doubtful, although it was the one under which the 

 vaiiety was bought several years ago from a leading 

 Boston nursery. Our clematis is different from the one 

 grown by Mr. Dwyer, although these differences are in 

 points of minor importance. The flower stamens are 

 longer and more slender in our variety, but the main 

 rlifference is in the greater height to which our clen\atis 

 grows. We have an old plant trained up against a 

 stable which reaches a height of twelve to fifteen feet, 

 most of which wood lives over winter. This is very un- 

 usual for clematis stans, and marks our variety some- 

 thing abnormal. It seems to me that ours ought to 

 have a conuuercial nursery name. It is certainly worthy 

 of separate propagation and distribution. 



Yours truly. 



/^^^ 



