Size variation in the vascular cambium and its derivatives in two Alstonia species

Two tropical tree species viz. Alstonia venenata Br. and Alstonia neriifolia Don. (Apocynaceae) were investigated to detect size variation in different elements of the cambium and its derivative tissues. Although these two species were grown under identical climatic and edaphic conditions, fusiform initial dimensions and the elements derived from them were larger in A. venenata than in A. neriifolia. Ray initials are rectangular in A. venenata but isodiametric in A. neriifolia. An appreciable increase in length was observed in the phloem and xylem ray cells when compared to the mother cells. Maximum elongation was observed in xylem fibers during differentiation from the respective fusiform initials.

FROST RING FORMATION IN THE STEMS OF SOME CONIFEROUS SPECIES

Seedlings of several conifer species were artificially subjected to freezing temperatures. Microscopic examination of sections, taken at intervals after the frost, revealed the way in which frost rings developed. Differentiating tracheids and xylem mother cells were killed by the frost, leaving a permanent band of underlignified and crumpled tracheids inside a band of dead cell tissue. Most of the cambial initials remained alive but developed abnormally into short irregular tracheids. Parenchyma cells proliferated mainly from the xylem ray cells. With subsequent growth, the growing stresses, which had become subnormal because of the collapse of killed cells, were restored. This was accompanied by the reestablishment of the cambium to its normal form.

A CONTRIBUTION TO THE CONCEPT OF WOUND REPAIR IN WOODY STEMS

Those xylem ray cells closely associated with the cambial zone will unite in proliferating with other cells recently derived from the cambium following the wounding of a basswood stem. However, ray cells remote from the cambium, although potentially meristematic, will fail to divide. Nonetheless these latter will grow out occasionally when the ray, exposed during the culturing operation, is closely connected with actively growing callus tissue. Parenchyma throughout the body of the secondary wood of the basswood has been shown to proliferate whenever a mass is exposed which is considerably larger in volume than a normal multiseriate ray. The healing of wounds in the linden best should be considered as a function of active, newly formed, cambial derivatives and not as a reaction dominated by any one tissue.