8 STUDIES IN HISTOLOGY. 



promptly produced. This test is preferable in some respects to 

 the solutions of iron alluded to in 59. 



Intercellular spaces of various shapes and sizes containing 

 air, or air and water, are met with in many of the plants already 

 enumerated. The most interesting are found in monocotyledo- 

 nous plants, notably Aracese and Juncacese. 



CELL-CONTENTS. 

 I. PROTOPLASM. 



No better material for the demonstration of the physical and 

 chemical properties of protoplasm in its active state can be em- 

 ployed by a beginner than the young stamen-hairs of Spiderwort. 

 Several garden species of Spiderwort are available for this pur- 

 pose, especially Tradescantia Virginica and pilosa. The green- 

 house species can also be employed. If none of these are at 

 hand, an}- }*oung large plant-hairs with thin transparent walls 

 will answer for the demonstration. If the hairs are sufficiently 

 young, the protoplasm appears as a nearly transparent mass 

 filling the cell-cavity ; but even when they are onty slightly 

 advanced in development the mass becomes honey-combed by 

 sap-cavities or vacuoles. With further development these be- 

 come confluent, and traversed here and there by slender threads ; 

 the wall of the cell, however, as long as it is alive, being lined 

 by a delicate film of protoplasm. 



When the protoplasm exists in a cell only in the latter condi- 

 tion, it is well to place the cell in a solution of sugar (a five per- 

 cent one will answer) or in dilute glycerin. By this means the 

 protoplasmic lining is contracted somewhat b}- the withdrawal 

 of water from its cavity, and in shrinking from the wall its shriv- 

 elled contour can be easity distinguished. 



It is best for a beginner to use in his early demonstrations 

 very young plant-hairs in which the vacuoles do not occupy much 

 space within the cell. The cells composing the growing points 

 of most roots, stems, and leaves are too small for satisfactory 

 study at the very outset ; it is well to defer the examination of 

 the protoplasm in these until its reactions have been clearly 

 demonstrated in young plant-hairs. 



Directions for the demonstration of active protoplasm can be 

 found in section 124. The tests there given should be repeated 

 by the student four or five times with different kinds of cells. 



