330 THE GARDENER. [July 



cells of this tissue that sap rising from the roots passes to the leaves ; 

 and as each new growth or formation of cells is made exterior to tho 

 alburnum periodically — not necessarily yearly — there will be a new 

 layer of growth, which will indicate one period of growth, whether 

 of one year or less. In tropical climates, if the trees shed their 

 leaves three times in a year, or five times in two years, we should 

 expect to find as many layers of wood ; the innermost tissue of the 

 alburnum becomes in its turn solidified and hardened by deposit, and 

 in its turn becomes heart -wood or duramen. Then comes the 

 cambium, or the cambium layer, variously described in its earlier 

 stages as " semi-fluid mucilage," " viscid secretion," " cambial or for- 

 mative tissue," " opaque fluid," or " mucilaginous semi-fluid matter ; 

 but it is cellular tissue of delicate texture filled with assimilated 

 granular matter in a state of vital activity forming new cells ; and 

 in this tissue the primary vascular tissue, or bundles, as they are 

 sometimes called, originate : here all the changes of the new growth 

 take place, and with it the leaves seem to be in connection : it is 

 found even at the point of the stem (punctum vegetationis) under the 

 bud. Part of it goes to form the new woody fibre of the alburnum, 

 and to continue the medullary rays in the direction of the bark ; other 

 part of it goes to add to the internal bark or liber ; and other part 

 separates the alburnum and the bark. The bark may be said to con- 

 sist of three principal layers of cell-tissue — the inner one of fibro- 

 vascular tissue, called the endophlceum or liber, the bast layer ; the 

 middle layer of cells, differing from the external layer, called the 

 mesophloeum ; and the external or corky layer, epiphloeum ; and then 

 the epidermis. The pith wood and medullary rays may be seen in 

 the herbaceous stems of an exogenous plant ; and there are medullary 

 rays, but no pith, in the roots of exogenous plants. JSTo hard and 

 fast rules can be laid down by man which will always explain the 

 works of nature, and accordingly there will be found among the 

 exogenous plants exceptions to our explanations of the formation 

 and arrangement of its tissue. In the Calycanthea3 (Caroline All- 

 spice family) there are some species which form woody columns 

 in the bark, independent of the central woody formation ; and in 

 some climbing plants a greater thickness of the medullary rays than 

 usual will be found. In dicotyledonous plants the division of the 

 stem into nodes, with internodes or spaces between, is taken advan- 

 tage of by the gardener to increase the number of plants : this he 

 does either by making cuttings of a portion of the stem at a leaf-bud 

 just below a node, which, when placed in soil and warmth, will soon 

 emit roots ; or by layering the stem or branch to the earth, fixing it at 

 a node to the ground, and making an incision with a knife into the 



