6 SMITH: THE HISTOLOGY OF CERTAIN ORCHIDS 
Lloyd (1919), in a paper on mucilage secretion in the cacti, 
the Malvales and Astragalus gummifer, concludes that the slime 
is in each case a product of the primary cell wall. He considers 
the relation of slime to growth as very important since the 
early differentiation of slime cells in the region of the growing 
point indicates a mechanical function connected with the growth 
process, the slime cells constituting an imbibition apparatus 
which forces the membranes to stretch and thus acts as a 
factor of growth. 
Miss Stewart (1919) has also investigated mucilage secretion 
in certain cacti. She concludes it to be a product of the proto- 
plast since the middle lamella of the wall between the cells 
which bear mucilage and those which do not is equally distant 
from the protoplast in both cases, showing that the cell wall 
has remained unchanged in the slime cell. Her conclusion as to 
the relationship of mucilage to growth agrees with Lloyd’s. 
In her description of the development of a mucilage cell she 
shows that the enlargement of the cell in the early stages is 
accompanied by increased density in the cytoplasm, and that the 
nucleus becomes larger, so that not only is there an increase in the 
volume of the cell, but the mass of material other than water 
is also augmented. As the slime cell becomes mature this 
dense cytoplasm with the nucleus is crowded to the centre of 
the cell and appears disorganized or may even disappear. 
Czapek (1913 and 1921) recognizes:— 
1. The slime which covers the epidermis of seeds, such as in 
Linum, the Lythraceae, and Plantago. This slime, he considers, 
arises from secondary thickening of the cell wall. It may exude 
from the cell upon the epidermis or into intercellular spaces. 
2. The slime which arises from a specialized layer between 
the cell wall and the cytoplasm. 
3. The slime which arises in the protoplast. Cells bearing 
raphides and the slime cells of various species of orchids belong 
to this class, which has been called cell content slime or mucilage. 
Czapek considers the protoplast slime cells in most of the epiphy- 
tic orchids as water containers. In the underground tubers of 
orchids as well as in the slime endosperm of many leguminous 
plants he looks upon these slime cells as stores of reserve car- 
bohydrates. 
