196 Miss Dale, On certain outgrowths (Intumescences) 



At the apex of the outgrowth are the guard cells of a stoma, 

 which although of large size leads into no definite cavity. 



A longitudinal section of an outgrowth containing chlorophyll 

 is seen in fig. 7. The palisade cells (since in this case the 

 outgrowth is on the upper surface of the leaf) are prolonged 

 between the basal cells of the outgrowth, with which, as well 

 as with one another, they are, at least in their upper parts, and 

 usually for their whole length, in uninterrupted contact. In the 

 same figure, to the right of the outgrowth is represented a 

 slightly raised stoma. Between such a raised stoma and the 

 larger outgrowths there are, in the leaves, all intermediate forms. 

 Although in general there are no cavities whatever, a few, 

 especially of the smaller outgrowths, have a central lumen which 

 places the stoma in communication with the cavities of the 

 mesophyll. In the case of the larger branched outgrowths there 

 is a stoma at the apex of each branch, and some also possess 

 stomata near the base. 



A transverse section near the apex of an outgrowth of median 

 size usually shews about four thin-walled cells in close contact 

 (fig. 8). They are thinly lined with finely granular protoplasm 

 and contain numerous leucoplasts, and, frequently, oil-drops. The 

 section does not pass through the nuclei, because, as the longi- 

 tudinal section shews, these lie about half way down the elongated 

 cells (figs. 6 and 7). 



In fig. 9 is represented a transverse section of an outgrowth 

 in which there is a narrow but distinct lumen. 



Figure 10 shews a transverse section of an outgrowth which 

 contains a prolongation of the mesophyll, and which also has a 

 cavity. It will be seen that the cells containing chloroplasts are 

 much smaller in diameter than those which are colourless. 



Figure 11 is a transverse section of an outgrowth in which all 

 the cells, some coloured and others colourless, are in uninterrupted 

 contact. This type is the more usual. 



Figure 12 is a cross-section (i.e. parallel to the surface of the 

 leaf) of a part of the green cushion from which arise some of the 

 larger outgrowths occurring in tufts. The internal, chlorophyll- 

 containing cells resemble in shape those of the palisade tissue, but 

 their walls (which are of cellulose) are very thick. The section 

 shews that the walls are strongly pitted. 



/3. Outgrowths on the Stem. These are all of large size, and, 

 in their younger stages resemble the largest of those found on the 

 leaves ; i.e. those with a thickened base, and are formed in the 

 same way. 



In the course of their history the outgrowths pass through 

 three distinct stages which may be described separately. The 

 normal cortex of a young stem is limited by a one-celled epidermis 



