12 STUDIES IN HISTOLOGY. 



or even almost black. The student should examine the very 

 peculiar globules of tannin-solution found in the sensitive pulvi- 

 nus, or cushion, at the base of the petiole of Mimosa (Sensitive 

 plant). Similar globules have been detected in different barks. 



DISTRIBUTION OF THE HISTOLOGICAL ELEMENTS. 



The various histological elements after being examined as 

 directed in Chapter II. should be investigated with regard to 

 their mutual relations. It is advisable to begin with the skele- 

 ton or framework of the plant, afterwards taking up the latex- 

 cells, etc. 



As shown in Chapter III., the framework of the higher plants, 

 which we are now to consider, consists of fibro- vascular bundles 

 variously arranged and conjoined. The bundles, which in some 

 cases may run for some distance as isolated threads, and in 

 others exist as compact masses, are surrounded with larger or 

 smaller amounts of cellular tissue, the exterior portions of which 

 are specially adapted to come into contact with the surroundings 

 of the plant. 



I. STRUCTURE OF FIBRO-VASCULAR BUNDLES. 



For the demonstration of the structure of fibro-vascular 

 bundles, seedlings of the following plants will afford good 

 material : Bean, Indian corn, Castor-oil plant, and Squash. The 

 roots of these plants give examples of radial bundles (313), in 

 which the strands of liber and of wood are in different radii, 

 while from their stems (including the hypocotyledonary stem of 

 the bean, castor-oil, and squash) may be obtained excellent 

 illustrations of collateral bundles. 



The sections for displa3ing the structure of the bundles are 

 best made in the three directions, transverse, vertical-tangential, 

 and vertical-radial. In a few cases sections made obliquel}- to 

 the axis of the organ are instructive ; but unless great care is 

 exercised in observing all their relations, the}' may be rather 

 misleading. 



In all cases examine fully the character of the bundle-sheath 

 (see 212). The student should not be satisfied with anything 

 less than a clear interpretation of all the structural elements 

 which he meets in a given bundle. If the structure of a bundle 

 is not revealed by the sections already prepared, fresh ones 

 should be made and carefully compared with the others, and 



