148 Introduction to Botany. 



fications to insure cross pollination (flowers of different ages are 

 necessary to show all that this flower has to teach) ; Oxalis, to show 

 special devices for cross pollination, each student being supplied with 

 the two forms of Oxalis violacea, for instance, which may be found 

 growing together in the same patch ; all obtainable species of violets, 

 to exhibit special devices for cross pollination, and considerable varia- 

 tions in the foliage and habits of plants having flowers of essentially 

 identical structure. This comparative study of violets is excellent in 

 showing how the flower, more than any other structure, gives the most 

 reliable clews to plant relationships. 



It is best to have the whole class at one time working on the same 

 kind of flower, in order that the discussions and blackboard demonstra- 

 tions, which are frequently desirable, may be founded on the experience 

 of all of the students. The material should therefore be chosen in time 

 to have an abundance provided for each day's work. 



OBSERVATIONS. 



119. Make a drawing of the flower, showing its position 

 on the branch and its relation to outgrowths from the 

 branch (a, Fig. 74). 



1 20. Dissect the flower into its separate parts, and make 

 sketches showing the form of each (</, f, g t h, i, /). 



121. Make a drawing of the flower on a larger scale, if 

 necessary, showing the sets of organs in right number and 

 proportion (b). 



It is best to sketch in lightly the form of the flower 

 before drawing its details. This can be most easily and 

 accurately done by drawing first a circle, ellipse, square, 

 or rectangle as a form guide, for most flowers conform in 

 general contour more or less to one of these figures (see 

 diagrams of Fig. 75). A flower whose general outline is 

 circular when held before the observer with axis horizontal, 

 as in H, will appear more and more narrowly elliptical as 

 its axis is shifted toward the vertical, as in / and J. The 



