92 MASS. EXPERIMENT STATION BULLETIN 190. 



HISTOLOGY OF THE TWIG IN RELATION TO ROOT 

 FORMATION. 1 



Roots on the scion usually arise near a bud, either singly or in twos or 

 threes. No case has been observed when roots arose at a node opposite 

 the bud. Roots may also arise from the internode, but generally within 

 a half inch of the node. Generally they arise above rather than below 

 the bud. The first indication of the root is the falUng away of the axillary 

 bud and the appearance of a swelling with two or three brownish white 

 areas, — the growing points of the young roots. 



Free rooting varieties develop roots early in the season. An examination 

 of Bough grafts in July showed that they were rooting freely. At the same 

 time Red Astrachan, Ben Davis and Tompkins King showed incipient root 

 formation in a few cases, while poor rooting varieties showed no signs of 

 roots. An examination about the middle of October showed progress in 

 aU these varieties, but the poorer rooting varieties showed hardly a tree 

 with roots from the scion. Always, on digging, the poor rooting varieties 

 have small roots (see Fig. 3) which have evidently formed the second 

 season of growth. 



If we examine a cross section of a one-year-old twig we find between 

 the bark and wood the cambium, consisting of a layer of eight to fourteen 

 very small, thin-walled rectangular cells. Measurements of the thick- 

 ness of the cambium layer were made and the number of cells noted on a 

 number of the varieties used. Measurements of the thickness of the 

 bark were also made. 



In choosing material, fresh twigs of the previous season's growth, 

 from both bearing and nursery trees, were selected, and cross sections 

 made usually at the fifth node back from the terminal bud. In the case 

 of some immature tips it was necessary to go further back to secure a 

 plump, mature bud. Sections were made with a sliding microtome and 

 placed at once in 30 i^er cent alcohol for ten minutes. Then the alcohol 

 was poured off and the sections stained for three to five minutes with 

 Delafield's Hematoxylin, washed, mounted on the slide and measured 

 at once. Measurements of the bark were to the wood, and included the 

 cambium layer. They were made at a point one-fourth around the 

 circumference of the twig from the bud when possible, and in aU cases 

 care was taken to avoid the thickened bark near the bud. The limits 

 of the bark as thus defined were clear, but more difficulty was experienced 

 in measuring the cambium layer because of a less clear differentiation 

 between it and the phloem. Often there are two or three cells that have 

 no distinctive features of either cambium or phloem. In order to estab- 

 lish a limit the phloem was considered as starting with the first cell, in 

 which the cells were markedly larger and more rounding, with walls less 



1 This discussion is based on work by Robt. P. Armstrong, graduate assistant, to whom the 

 credit for it is due. 



