104 



THE LEAVES OF PLANTS. 



En it, therefore, we rather find the place and the nidus in which the leaf will be 



formed in the coming Spring than the parts of a leaf in a rudimentary condition. 

 There are only two parts which need attention the central 

 growing point and the imbricated scales (Fig. 187). 



The growing point is composed of cellular tissue, pos- 

 sessing special powers of vitality and growth, and connected 

 with the horizontal system, or pith of the stem. There are 

 no vascular structures within the point itself, but spiral 

 vessels and woody fibre approach near to its base (Fig. 105 a). 

 It has a highly important function to perform, for not only 

 is it the point from which all the future leaves must be 

 developed, but it is probably the means whereby the circu- 

 lation of the sap of plants is again effected after the quietude 

 of the previous winter. To what anatomical part of the 

 growing plant this " pumping " power is to be attributed is 

 unknown, find the vital principle which excites it to action 

 has not' been discovered ; so that we must at present regard 

 this property simply as being a part of its constitution, and 

 of that of the plant as a whole. This growing point has a 

 certain analogy with the embryo in the seed; inasmuch as both 

 tend to growth and reproduction ; but they differ inasmuch that 

 the leaf-bud needs no fertilization for its development, and 

 propagates the individual as well as the species, whilst the 

 embryo imperatively needs fertilization, and continues the 

 species, not the individual. There is also a resemblance be- 



tween leaf-buds and bulbs, page 71. 



The imbricated scales (Fig. 187 ), are called tegmenta or coverings, since their 



duty is to protect the delicate growing point. 



They are foliaceous organs, and are con- 



sidered to be identical with stipels. The 



outer ones are usually harder and of ruder 



texture than the inner ones or those more 



immediately surrounding the growing point ; 



and in cold climates a further protection is 



afforded by a thick downy covering, as in 



willows, whilst the scales are thinner and 



smoother in plants growing in tropical 



regions. All the scales, at least in many 



plants, are ultimately developed into 



leaves. 



The normal position of a leaf-bud is in 



the axil of a developed leaf ; but, according to 



the opinion of certain physiologists, the sap 



Fig. 187. The leaf bud, 

 with its imbricated 

 scales, &, pointed extre- 

 mity and cicatrix of old 

 leaves, a. The growing 

 point is inclosed and 

 hidden by the scales. 



18?. The leaf O f the Bryophyllum cagci. 

 num, in which leaves are developed at its bor- 

 ders. 



has the power of producing buds in any 



*, & . . , J 



position. It is well known that they have 



been produced upon the stems of plants and 



upon the leaves of the Bryophyllum (Fig. 188) ; and the fleshy parts of most plants, aa 



of the bulb of the Hyacinth, may, by care, be compelled to produce buds, and to repro- 



