292 



METAMORPHOSIS 



spasmodic variations in the growth of Bambusa are very remarkable (Kraus, 

 1895) as may be seen from the following curve (Fig. 86). 



We can distinguish, finally, in the leaf during its embryonic growth, as 

 a rule, two regions, a proximal and a distal. From the distal region is developed 

 the blade, from the proximal part arises either a leaf -sheath or merely a flattened 

 point of attachment to the stem, which may become enlarged into a well-dif- 

 ferentiated structure, 'the pulvinus'. In accordance with the area of the leaf 

 attachment to the stem, in so far must it elongate with the stem, and thus we 

 see the growing stems of Coniferae densely covered with pulvini, as is specially 

 evident in the spruce itself. In all cases where the leaves are thus developed 

 close together, the leaf-bases must accompany the stem in its elongation as in 

 the Coniferae. It is true one very often sees nothing of them externally, and 

 distinct pulvini may be entirely absent. All the same a careful investigation 

 will show generally the existence of such where a free stem-surface between the 

 leaf rudiments does not exist from the outset at the growing point. 



Between the leaf -base and the blade there appears frequently in the growing 



Fig. 86. Daily growth increments (in cm.) in the stem of Batnbusa, measured at Buitenzorg from Nov. 13, 

 1893, to Jan. 10, 1894. After Kraus (1895, PI. 20). 



leaf a very distinct region, the leaf-stalk, which arises generally by intercalary 

 growth in a zone of tissue of minimum extent between the leaf-base and the 

 blade, but only after the blade has progressed considerably in its development. 

 We learned previously that at the beginning the blade also shows apical 

 growth. Only in a few cases does this apical growth continue for long, generally 

 it ceases long before the rudiments of all the parts are complete, or at least 

 before their elongation commences. Among ferns, Gleichenia and Lygodium 

 are known to possess leaves with apical growing points which remain active for 

 years, and even among our ordinary ferns it may happen that new pinnae 

 are developed at the growing point when the basal ones have already opened 

 out. Similar cases occur in Phanerogams, as we learn from the researches of 

 Raciborski (1900), who found that in certain of the Meliaceae (Guarea, Chiso- 

 cheton), the power of producing new pinnae at the apex of the leaf was long 

 retained. According to Sonntag (1887), the leaf of Guarea possesses only a brief 

 apical growth, during which a limited number of pinnae are laid down, which 

 expand partly in the first, partly in the second vegetative period. The supposed 

 likeness to ferns thus breaks down, or to speak more accurately, it is limited to 

 a slow and purely acropetal expansion, which occurs also elsewhere. 



Frequently we meet with another type of leaf-expansion where the apex 



