WENHAM, ON THE SAP-CIRCULATION OF PLANTS. 45 



may be seen traversing straiu^lit from the base towards t'le 

 apex of the hair, and returning again by the opposite side. 



The circulation in the hairs of the Groundsel (Senecio 

 vulf/arU) was first announced by Mr. Holland, as a discovery 

 made by his triplet microscope, and it is a remarkable instanc e 

 of what his instrument was capable of performing, for out of 

 the multitude of vegetable hairs in which the sap-motion can 

 be seen, I consider this to be one of the most difficult ; for 

 with our best compound microsc;opes, it requires careful ma- 

 nagement, and a trial of several fresh specimens before it can 

 be shown satisfactorily. 



Hairs that exhibit circulation may be taken from all parts of 

 the plant, as the leaves, flowers, stalks, and fruit, and even from 

 the ripening seed-pods as in the Snap-dragon {AntirrJiiinmi) 

 and White Mustard {Sinapis alba), &c. It is important that 

 the specimens should be gathered from a portion of the plant, 

 in a healthy and vigorous state of growth. The time is also 

 of some consequence, the motion of the sap being generally 

 most rapid about mid-day. The specimen must be examined 

 as soon as possible, and the hairs detached without touching 

 them, by tearing them off with a portion of the cuticle of the 

 plant to which they are attached, hy means of a fine-pointed 

 forceps. If the hair itself is grasped the destruction of its 

 vitality is the usual consec^uence. The object should be 

 instantly placed in a thru glass compressor with clean water, 

 using a good eighth object-glass and an achromatic condenser 

 having a series of diaphragms. Daylight is infinitely 

 superior to artificial illumination, and I have found it much 

 preferable to use a right-angled prism instead of the ordinary 

 plane mirror. 



In cold dull weather, a well-known ol)ject will sometimes 

 fail to exhibit its circulatory movements ; in such a case, it 

 may be called into activity by means of the natural stimulus 

 of heat. In applying this the object need not be removed 

 from the microscope, as a stream of hot air may be blown on 

 to the upper or under surface of the thin covering glass, 

 until the sap current is seen to move, by means of a metal 

 blowpipe, or the stem of a tobacco-pipe, previously heated in 

 the flame of a spirit-lamp. Some plants always require 

 the application of an increased temperature, in order to show 

 their circulatory movements. The hairs of the Helianthus are 

 a good example of this. 



In the hairs of the numerous variety of plants that display 

 the sap-circulation, each species exhibits somewhat different 

 and peculiar features, which may be considered, in a degree, 

 characteristic throughout the plant ; in some, single lines of 



