130 



LECTURE VIIT. 



freezing and thawing, and washing out in the rain. Of course such skeletons 

 can only be obtained if the vascular bundles are lignified, or surrounded with 

 lignified sheaths; since these, however, are found in the most various subdivisions 

 of vascular plants, it is easy to make a collection of vascular bundle skeletons, 

 certainly among the most instructive objects that can be preserved in a botanical 

 museum. The careful and repeated study of successful vascular bundle skele- 

 tons of the most various leaves and shoot-axes, as CEnanthe phellandrtum, Zea 

 Mais, or old withered stems of Palms and Dracanas, as well as the fruits of the 

 Thorn-apple {Datura Stramonium), old Banana fruits, and the like, is a most 

 attractive occupation, provided that the observer is to a certain extent familiar with 

 the physiological significance and origin of these structures. Only to one possessing 



a sufficient knowledge of these mi- 

 croscopic relations can the micro- 

 scopic structure of the vascular 

 bundles be intelligible ; and in order 

 to arrive at clear conceptions con- 

 cerning the conduction of materials 

 which devolves upon the vascular 

 bundles, and their co-operation in 

 ensuring the solidity of the plant, the 

 study of vascular bundle skeletons is 

 indispensable. Their significance in 

 teaching is even in our time much 

 too little appreciated. 



It is not very easy, in the short 

 space at our disposal, to give a clear 

 insight into the microscopical struc- 

 ture of vascular bundles: this, as 

 need hardly be mentioned, undergoes 

 great variations in the different sub- 

 divisions of plants. Only the most 

 necessary points will be mentioned 

 here, since I must repeatedly refer 

 in detail to more special matters 

 subsequently, when we are concerned, 

 with physiological functions. In the first place, it is to be observed that a bundle 

 by no means maintains the same structure through its entire course ; the lower 

 and upper ends are generally thinner than the middle parts, and therefore more 

 simple in structure. As the thickness — i. e. the surface of the transverse section — > 

 grows, not only the number but also the variety of cell-forms increases. Thicker- 

 bundles may exhibit hundreds of cells on the transverse section, while the thinnest 

 (e. g. in the venation of the leaves) ,consist of a few cells only. 



It may also be pointed out that the microscopic picture of the vascular bundle 

 in trarisverse section is generally extremely characteristic, and easily impresses itself 

 on the imagination ; wherfeas, in consequence of the great length, frequent thinning: 

 out, an4 oblique course of the individual elements, the picture of the longitudinal 



FIG. \^^~Cyathea Imrayana (a Fern). Portion of a stem with the 

 bases of four leaves, after the removal of the cortex, and showing the 

 course of the vascular bundles. The dark bodies are roots. (After De 

 Bary.) 



