822 Yapp . — Spiraea Ulmaria , Z., and its Bearing on the 
diminished hairiness in the lower leaves. But Mr. E. J. Salisbury informs 
me that in the wild Raspberry, when the canes are growing in thick clumps, 
the lower leaves are considerably less hairy. Specimens kindly sent by 
Mr. Salisbury confirm his observation. 
Thus we are evidently dealing with a widespread phenomenon. 
§ 7. Seasonal Differences in Leaf Structure. 
Various authors have called attention to the periodicity of growth ex- 
hibited by shoots. 1 The leaves increase in size and the internodes in length 
(and also in the length of their cells) from below upwards. Such increase is 
followed by a corresponding decrease after the maximum is reached (cf. 
PI. LXXXI and Text-fig. 5). It has been shown above that a similar 
periodicity of hairiness obtains in the non-flowering shoots of Spiraea 
Text-fig. 3. Sections of (a) a ‘ sun ’ leaf, and (b) a ‘ shade ’ leaf of Spiraea Ulmaria. x 350. 
Ulmaria. The leaves of the erect flowering shoots, on the other hand, 
exhibit a more or less continually rising curve of hairiness. It will now be 
shown that corresponding seasonal differences of structure occur in the 
successive leaves formed on these erect flowering shoots. 2 
The possible range of structure may first be indicated. Text-fig. 3 
shows sections taken through extreme ‘ sun ’ and ‘ shade ’ leaves. A is from 
a cauline leaf of a fully exposed plant, and B from a radical leaf of a plant 
grown in deep shade and a constantly humid atmosphere (under a bell- 
glass). 3 
1 Moll (76), Pfeffer (’03), Tammes (’03), Groom (’08), &c. 
2 The method of investigation was as follows : The successive leaves on selected shoots were 
preserved in alcohol, care being taken not to remove any leaf from the plant till it was fully mature 
and several later leaves had unfolded. Sections were taken from corresponding portions of the terminal 
leaflet. Another portion of the same leaflet was cleared in eau de Javelle and mounted in water as 
a transparent object. In this state the size and relations of the epidermal, palisade, and other cells 
could be studied, and the number and size of the stomata ascertained. The cells remain of approxi- 
mately the same size as during life. The latter point was determined by taking a contact print of 
a living leaf on photographic paper. The leaf was then treated as above, when it was still found to 
correspond in size with the photographic print. Thus the number of stomata per unit area given 
later may be taken as practically the same as in a living leaf. 
3 Cf. the figures given in Hesselman (’04), p. 419. 
