REVIEW, \65 



al embryo, while lie denominates that within the seed a free embryo. 

 Thus the wood and bark are considered as formed of the roots of the buds 

 which are annually developed on the surface of the vegetable. 



" Whatever may be the diode in which the formation of the new layers 

 takes place, it is known that the matter which forms them descends from the 

 leaf-buds or leaves, either in the innermost layers of the bark, or between 

 it and the alburnum. 



"If all the bndfi or leaves be removed from the upper part of a branch, no 

 increase in diameter will take place above those that are left. If a ring of 

 bark he removed from a tree, the part below will not increase in thickness, 

 and the upper lip of the wound will heal quickly, while the lower lip will 

 not. This operation lias been recommended for improving the fruit of trees ; 

 • lie descending sap or proper juice, confined to the upper part, increases 

 the size of, and enriches the (lowers and fruit developed above the place from 

 which the ring has been removed. This is called ringing : care must be 

 taken to make the ring very narrow, in order that the parts may easily 

 re-unite. 



"If a ligature be placed tightly round the bark of a tree, the part above 

 the ligature will swell, but not the part below; and it has been observed 

 that the rate of increase of the diameter of any part of a branch or tree is in 

 proportion to the number of leaf-buds developed above that part. 



" From their peculiar structure, Exogenous stems readily throw out 

 branches, and hence the form of the trees in this country is so different from 

 that of the Palms. Figure 5 shows the general outline of an oak tree. The 

 part of (lie tree where vegetation is active being near the outer surface, (he 

 buds easily penetrate and grow into shoots, from the sides of which also buds 

 are developed, and thus the tree is branched and subdivided to a great ex- 

 tent, and from a short distance above the ground. 



" Prom the same cause, such trees attain a great thickness and live to a 

 great age. The new matter being added externally, has little resistance t<> 

 overcome (only that of a thin bark, which is easily distended and pushed 

 out) and thus there is hardly any limit to its increase in diameter, while, 

 becoming only more securely fixed as it grows, and the growth going on 

 between the bark and the wood almost independent of the inner parts, there 

 is as little limit to the d oration of the tree. In fact each annual layer of 

 alburnum or bark si ems to have an independent existence — hence trees are 

 often found flourishing, though quite decayed and hollow within. 



PROPAGATION OP PLANTS. 



"There is a very large assemblage of plants which produce their seeds by 

 stamens and pistils, and they are called flowering or phsenogamic plants In 

 most cases the stamen and pistil are together (in the same flower), the Bower 

 being then called perfect. 



" In some plants the stamen and pistil, though on one plant, are not to- 

 gether, as on the oak and the nettle. 'I he flowers are then said to be mono- 

 ecious (par. 216), and in these eases the pollen either, falls on the stigma, or 

 is conveyed to it by the wind, or by insects. 



"In other plants, as the willow, tie' hop. one plant has stamens only, while 

 another 1 as pistils only. The flowers in this ease are called Dioecious, and 

 the same means serve to convey the pollen as in the last case. A Bower 

 with pistils only is called Pistillilcious ; one with stamens only is called 

 Anteriferous, 



"In the following tribe of plants the pollen must reach the stigma in order 



that tl Miles may ripen and become seeds, and there are main different 



ways in which this is brought about, 



" In a great number of cases the flower is erect, the stamens are longer 



than the style, so that the anthers are above the level of the stigma : and 

 whin the celll of the anther open, the ripe pollen qi re.ssaiily falls upon the 



stigma. In other cases, where the pistij is longer than the stamens, the 



