CHAPTER III. ORGANS OF PLANTS. 1 99 



or less frequently, solid colored corpuscles. The velvety appear- 

 ance of some petals and sepals is due to the conical form of the 

 epidermal cells, which are so arranged that the apices point 

 outward. The fibro-vascular system of floral organs is also 

 usually simpler than that of ordinary leaves. The minute 

 structure of the essential organs of the flower has already been 

 sufficiently described in Part I to answer the purposes of this 

 work. , 



Practical Exercises. 



1. For the study of open collateral bundles, make sections transverse and 

 longitudinal of a stem of Menispermum Canadense, selecting one that has 

 attained a season's growth. This is selected for the first study, because the 

 fibro-vascular bundles are sharply distinct from the other tissue systems, the 

 medullary rays being broad, unlignified and composed of thin-walled cells, and 

 the various kinds of tissues of the bundles are well developed. It also affords 

 a simple and easily understood example of the Dicotyledon type of stem struc- 

 ture. The sections should be cut thin, and care should be taken that the 

 longitudinal ones run as nearly lengthwise of the grain as possible. To get a 

 clear understanding of the structure, some of the longitudinal sections should 

 be radial and others tangential. A radial section, it will be remembered, is 

 one which passes through the middle of the stem lengthwise,— that is, along the 

 medullary rays, — while a tangential section is made to one side of the centre, 

 so as to cross the direction of the medullary rays. The sections used for imme- 

 diate study should be placed on a slide, treated with a drop of phloroglucin 

 solution, and then, after the solution has had time to thoroughly penetrate the 

 tissues, but before the liquid has evaporated from the cells, put on a drop of 

 chlorhydric acid, cover the section with a cover-glass and examine. The tissues 

 of the xylem, the bast cells of the phloem and other lignified tissues will, by 

 this process, be stained a bright red, while the unlignified tissues, such as most 

 of the pith cells, the medullary ray cells, the cortical parenchyma, the sieve 

 tissues and the cambium cells will not be stained. 



Some of the longitudinal sections should be treated with Schultze's mace- 

 ration fluid for the purpose of isolating the tissues and studying them in detail. 



Those sections which it is desired to mount permanently should first be 

 treated with alcohol, 70%, for several hours, and then double stained as follows: 

 Soak the sections for about half an hour in a solution of iodo-methyl-green ; 

 then afterward, for a similar length of time, in ammonia carmine solution ; then 

 they should be rapidly rinsed in distilled water, transferred first for a short time 

 to weak alcohol, and then, as rapidly as possible without causing shrinkage of 

 the sections, to stronger, and finally to absolute, or at least 98% alcohol. 

 Here they should remain for about half an hour, until nearly deprived of water, 

 when they are to be transferred — first, to oil of turpentine for a few minutes ; 

 then, for half an hour or more, to oil of cloves, and then, finally, monnted in 

 Canada balsam. Thus prepared, the tissues will be well differentiated, and 

 may easily be studied. 



