36 MR ALEXANDER MORTON ON 
and floral structures. Many cultivated species of cruciferous 
plants illustrate this feature. The foliage of trees, shrubs, 
and perennial plants exhibits a more constant character. 
The succession of leaves in our forest-trees is almost mvari- 
able from the time they burst forth in spring until they fall 
in winter. But when the external conditions do not admit 
of the full development of leaves, bud-scales are produced. 
The morphological interpretation of these organs is that they 
represent leaves or stipules reduced in structure and changed 
in function by the external conditions prevailing. They 
are well seen in the horse-chestnut. 
Stipules are found in pairs at the base of the leaf-stalk 
in many plants. They are thus leaf-structures, and are 
characteristic of several natural orders, as the Rosacez, 
Leguminos, Rubiacew, and Polygonace, while from other 
orders (eg., Cruciferee) they are absent. Stipules are 
interesting from a comparative point of view, as they 
exhibit striking changes in structure and function. In 
many of the Papilionacee there is a remarkable relation 
between their development and the metamorphosis of the 
leaf. In the pea, the stipules are larger than the lateral 
leaflets of the compound pinnate leaf. But the leaf of the 
pea ends in three tendrils—two lateral and a terminal 
unpaired one. These tendrils are the organs by which the 
pea-plant climbs, and they are morphologically leaflets. 
Now, as all plants practise a strict economy in the expen- 
diture of the organic substances they manufacture for the 
upbuilding of tissue, the pea-plant has found it an advantage 
to change these three terminal leaflets to organs by which 
it may climb, and consequently the stipules are more fully 
developed as assimilative organs. On comparing the bean- 
plant with the pea, the most obvious difference is seen in 
the fact that the former is able to grow upright without 
support, while the latter is unable to reach light without the 
help of its tendrils. This means, histologically, that the bean 
has developed a supporting tissue, and that the pea lacks 
this tissue. If the leaf of the bean-plant is examined more 
closely, however, the lateral leaflets are found to be equally 
developed, and the stipules are not so exaggerated as in the 
pea-plant. But at the end of the mid-rib a small vestigial 
