FLORAL DEVELOPMENT OF HELIANTIIUS ANNUUS. 125 



With regard, however, to the two leaflets or scales there are 

 certain facts which, as it seems to me, will prevent their reference to 

 the same class of appendages, whether we regard them as sepals 

 or not. 



" Trichomes," according to Sachs, " is the term given in the 

 higher plants to those outgrowths which arise only from the 

 epidermis, i.e., from the layer of cells which always remain the 

 outermost in roots, stems, and leaves." According to Warming, 

 they are of two kinds, viz., hairs, as we generally understand the 

 term, and a emergences," the last being the strongest, and some- 

 times containing a rudimentary vascular bundle. To this last class 

 the tentacles of Drosera would belong. 



Now with the definition just quoted, the structures under con- 

 sideration certainly do not agree, their origin being the same as that 

 of all leaves — an extension of the Dermatogem and an outgrowth 

 of the underlying periblem in the form of a tubercle. That they 

 have a broad base of insertion is shown in Fig. 6, the dotted line 

 representing their form and position at this stage of development. 

 It will be observed that their point of origin, as shown in Fig. 4, is 

 on a level with that of the carpellary leaves, and for some time this 

 position is preserved, but ultimately, owing to more vigorous growth 

 on the periphery, they are found surmounting the ovary (Fig. 9). 



At maturity they are leaf-like in form, with a serrated margin 

 (Fig. 17). In section, at their thickest part I find six or seven 

 layers of cells beside the epidermis. Up the centre from base to 

 apex runs a fibro-vascular bundle containing four or five spiral 

 vessels ; on either side there is another bundle, with a spiral vessel 

 or two for about one-third their lengths ; these anastomose with the 

 central one before reaching the apex. Outside these there are three 

 bundles, still more rudimentary in character, and extending but a 

 short distance into the lamina, in one of which there are no vessels. 

 Both stomata and chlorophyll are absent. 



Simple as is the structure of these leaflets, and far removed as 

 they appear from either leaves or sepals of the ordinary type, I 

 think it can be easily shown that we have a series of links, almost 

 complete, which will connect the two extremes with each other. 



In Figs. 18 to 22 are shown a series of bracts from one capitu- 

 lum, Fig. 18 being at the same time the innermost bract of the 

 involucre and the outermost floral one. Now between this and the 

 ordinary foliage leaf examples may at once be found which will 

 convince the most superficial observer that the one is but a modified 



