212 



REPRODUCTION, VEGETABLE (VEGETABLE OVUM). 



head of this article, is employed in order to 

 connect it with the preceding one on the 

 " animal ovum," to which it is intended to 

 form a sequel. In its u^ual acceptation in 

 vegetable physiology, the word means the 

 generative product of the Phanerogamia only. 

 And even if we were to extend its meaning so 

 far as to include all those varieties of germ, 

 for the development of which two organs 

 mutually dependent on each other for the ac- 

 complishment of their reproductive functions 

 are necessary, we should still he obliged to 

 disregard one half of the vegetable kingdom. 



PART I. 

 ALG/E, FUNGI, AND LICHENS. 



1. Reproduction by means of Z<-ospores. 

 Among the most simply organised infusory 

 animals are included several genera, which are 

 admitted by all naturalists to present, in the 

 aggregate of their characters, as many points 

 of resemblance with plants as with animals. 

 They agree with plants in their chemical con- 

 stitution, in the mode in which they react on 

 the atmosphere, and in their green colour. 

 The Euglena viridis, which is so common in 

 all our shady ponds, though in active motion 

 during the greater part of its life, manifests 

 at other periods a condition of plant-like 

 repose. The contractility displayed in its 

 rapid and ever-varying changes of form is a 

 property which, there can be little doubt, 

 manifests itself frequently among undoubted 

 plants ; * so that the transition from the Eu- 

 glenas to many of the forms of the Proto- 

 coccus-like Algae is almost insensible. The 

 elaborate researches of Colin on the so-called 

 Protococcus pluvialis, have unfolded many 

 facts of the greatest importance in relation to 

 this subject. The well-known permanent form 

 of this plant is that of a globular cell, furnished 

 with a distinct colourless membrane, and con- 

 taining in its interior a semifluid protoplasma, 

 in which numerous green or red granules are 

 embedded. Colin found that when water is 

 added to Protococci in this condition, they 

 immediately become the subjects of an active 

 reproductive process. In the interior of each 

 cell are formed, by the division of its contents, 

 secondary cell-like bodies, the number of 

 which is always either two, or a power of two. 

 These bodies, which possess no distinct mem- 

 brane, either give rise to stationary cells similar 

 to their parent, or, as is by far more frequently 

 the case, especially when the number of newly 

 produced individuals is large, they become 



* The presence of contractility of the substance 

 in true plants is still doubted by some physiologists. 

 One of the most accessible proofs of its existence is 

 to be found in the motions of the tapering growing 

 extremities of some species of Oscillatoria. Here we 

 have changes of form of the substance of the plant, 

 rapidly succeeding each other, and developed inde- 

 pendently of the action of any external stimulus. 

 These motions ma} 7 be observed with perfect facility 

 and occur under the most simple conditions. 



pear-shaped, fusiform, or oval ; at 

 time they are endowed with the 



Fig. 127. 



the same 

 power of 



Cell of Protococcus pluvialis, containing 



Zoospores, about 25 diam. (Cohn.) 



active motion, and are furnished with a pair 

 of vibratile cilia, emanating from their anterior 



Fig. 128. 



Free Zoospores of the same. (Cohn.) 



extremities. In the course of their further 

 development, these actively moving bodies, 

 which we shall call Zoospores, become invested 

 with a distinct membrane. This seems to be 

 a preparatory step to the cessation of their 

 movements; for shortly afterwards they are 

 observed to lose their vibratile cilia, and as- 

 sume a form which corresponds more or less 

 completely to that of the mother cell. In 

 many cases, however, before this result is ac- 

 complished, a second reproductive process 

 commences in the still ciliated zoospore. A 

 division of its protoplasmic contents, similar 

 to the first, takes place, and a second genera- 

 tion of zoospores is set free, each of which is 

 capable, after exhibiting active motion for a 

 longer or shorter period, of becoming a sphe- 

 rical, motionless cell, in all respects similar to 

 the original parent. Thus an individual Pro- 

 tococcus in its stationary form, may reproduce 

 itself either directly, or with the intervention 

 of a second generation. In the former case, 

 the germ may either become at once an indi- 

 vidual similar to its parent, or may pass 

 through a preparatory period, during which 

 it is not only provided with motor organs, but 

 manifests in the protoplasm of which it is 

 formed, a property of contractility resembling 

 that of animals. Facts similar to the above 

 are described by Braun as occurring in another 

 unicellular Alga (Ascidiumacuminatum). This 

 species, which is found attached to stones or 

 other objects, resembles the Protococcus plu- 

 vialis in its general form. By the division of 

 the protoplasma which lines its cell-wall, 

 numerous zoospores take their origin. These 

 are pear-shaped, and at the apex of each is 

 observed a pair of vibratile cilia. 



2. In the above-described plants we have 

 examples of the occurrence of zoosporous 

 reproduction under the most simple condi- 



